Marauder Semi-auto power tune mods

[Calson]

If you want more power than buy another rifle that has the power you want. It will be simpler and faster and no risk of having the gun fail. With the SAM rifle one is opting for rapid firing of up to 10 shots and the price is less power and a lighter pellet that can be used. I have the SAM in .22 and also a Air Arms in .25 caliber and if I wanted more power I would get something like the Air Force Texan in .30 caliber with muzzle energy of 275 ft lbs as compared to 29 to 38 ft lbs with the SAM.

With the Texan .30 rifle I would have a more expensive rifle and be using more expensive ammo and get far fewer shots between air fills (as in 5 shots per fill). For hunting out to 100 yards it will provide a great deal more hitting power. For target or small critters at up to 40 yards the SAM .22 is enough gun.

In the middle ground is a .25 caliber rifle that provides double the hitting power of a .22 caliber without the fill issues with a very large caliber airgun.

 

[Airgun-hobbyist]

Got the TP situation fixed. It is smaller than previous but didn’t want to drill off the ends again and end up stuck back there shooting orings. Now the the TP seems fixed and stable, cycling is a problem. Starting at about 2050 PSI the rifle will not cycle. It is not destroying magazines, it is now munching pellets. It seems to double load and jam every shot. Tried adjusting the hammer spring downward and quickly reached the place where the hammer would not catch. The bolt would cycle there, seemingly normally, without making the farting sound and double loading. It seems like more valve spring might help, but this is also past the point where it is possible to balance increased power and cycling. The shots that leave the barrel are flying 860-880 fps. 


Robert at Lane recommended 130 Bar or about 1850 psi to start, but if 2050 psi is running in the 880 ballpark and not cycling I’m not sure there is any reason to try lower pressure as that would just mean lower power, so not the goal. 


it seems the Crosman engineers developed the small plenum to keep the bolt/magazine and hammer cycling all coordinated. The Lane regulator is really nice but the increased plenum volume seems to create additional problems. There is an extra valve here to experiment with, but given the functionality of the higher PSI lower plenum volume with larger TP pathway it seems like going back to that might be the best bet. 


John if you have any ideas they would be very much appreciated as always. You already managed to get me pretty far down this path without anything going really off the rails so thank you again for that and thanks again to the OP for opening the topic and providing great feedback. 


If shimming the valve spring or the hammer spring might help let me know? It looks like everything else internally is working correctly. 


I’ll get 10 single shots as a string to get a better average and see what that looks like…

I had feed issues with 22 grain slugs. So both of our "issues" are cycling related in a way, not plenum space. The system needs to stay in some semblance of balance for it to operate as designed. Therefore the it would seem that the extra air volume is making it try to cycle twice in a single shot. Try the heavy ammo first as an experiment there to see if it still does it. If it does, then swap out by the transfer port to the original o-rings and .156" drilled port. These two changes will tell you if it is additional air volume effecting it. If it turns out to be the air, then the spring rates need to be adjusted to make the whole system work in harmony as designed.

You can go back to the medium sized .156" transfer port and stockish plenum and valve body, and have decent speeds with 15.89 grain pellets as a slightly modified SAM that cycles fast and reliable. This is where I am at with mine.

 

[FuzzyGrub]

Therefore the it would seem that the extra air volume is making it try to cycle twice in a single shot.

If your mag is actually indexing twice in one shot, please clarify.

 

[acmillerus]

The stock setup is a pretty incredible balance indeed. And thanks again, I am prone to jump to conclusions and got a little confused trying to understand the problem. Appreciate the restatement above, that helps. 

 Installed Lane reg at the recommended 1850psi setting, and power was the same? I assume the hammer and bolt were cycling fine at this point?

Installed Lane reg at recommended pressure about 1850 PSI. Didnt have the chrony up initially just wanted to see cycling. First couple shots made an abnormal bop-bop noise then would misfeed. Then cycling normally started. Put the Chrony on and saw some 850 fps numbers. Seemed reasonable that the larger plenum and lower PSI would be about the same as stock FPS, and turned the reg pressure up to 2050. The normal cycling is probably when the TP leak came into play. 

– Raised the reg pressure to 2K psi and power was still the same? Hammer and bolt cycling fine?

At about 2050 started getting inconsistent (but generally around 850, and NOT higher than stock plenum size as would be expected) FPS numbers. Cycling was consistent without feeding failures. Increased the pressure to about 2250 the pressure and saw a couple shots over 900 then dropping to 820. Some normal feeding and cycling and some intermittent feeding failures and jams started here. Seemed similar to the previous set of high PSI regulator experiments. 

– Opened TP to 0.170" and power was still the same? Hammer and bolt cycling fine? Then found broken TP part. 

The TP was drilled (or overdrilled) earlier for the stock reg testing. This resulted in a power improvement and some cycling problems at higher PSI. This TP was used until the broken part and gasket problems started after installing the same reg. Then tried the aluminum tube, then replaced that failed experiment with a new but redrilled crosman TP to ab out 0.155 . 

– Replaced the TP with Crosman or homemade TP? Opened to what ID?

Replaced the TP with a crosman TP drilled to about 0.155, gasket holding parts intact. 

– Now the cycling and hammer not cocking started? Does the 1st shot shoot fine, then misfires from then on?

The first shot makes fires but does not sound normal. I tried messing with the hammer spring settings to see if that would help. Decreased hammer spring tension and got a normal sound from the bolt but the hammer stopped latching. Turned the hammer spring tension back up near the max and the Bop-Bop-misfeed starts again consistently.

Dropped the Lane reg pressure back to 2050, but continued to get bop-bop-misfeeds and jams. Disassembled and reassembled a couple times and again everything appears fine, except I was noticing lead on the bolt face. The misfeeds were not chewing up the magazines, but were chewing up pellets, unlike the previous set of experiments. The seemingly random FPS numbers, from 800 to 900 continued until I found the TP gasket on the floor after one of the misfire/jam situations.

Now we are up to date. Reg pressure at about 2050psi and misfeeding every shot. Continue to get bop-bop-misfeeds and jams. Disassembled and reassembled a couple times and again everything appears fine, except I was noticed lead on the bolt face. The misfeeds were not chewing up the magazines, but were chewing up pellets, unlike the previous set of experiments.

Did you get a chance to look at the bolt and hammer lugs to see if there was any indication of them hitting each other. If either has started to back out, that would happen.

They both look good and dont appear to be making contact.

How does the barrel O-ring look? Probably worth replacing. -009, any pieces of pellets blocking the top of the TP? A piece of the broken TP or O-ring?

There are no peices of proken TP/Pellet/gasket in the TP path now but there certainly were previously. Sorry about all the confustion that caused, I should have stopped and asked for feedback when things did not look right!

The probe tip is intact but it is cutting into pellets as though it is sycling faster than the magazine can completely index, but not quite so fast it takes a bite out of the plastic? Maybe there is a clue in the pictures... 

Havent calculated the new plenum size but it is maybe 75% over the original size? 

The 10 single shot string looks like this:

Shot count: 10
Low: 879
Hi: 900
Avg: 888
Spread: 21
STD Dev: 6.8
891
891
879
900
898
884
881
891
891
881

 

[acmillerus]

 



The pop-pop-misfeed magazine indexing dilemma… 

it seemed clear in the much earlier experiments with the stock reg that the bolt was cycling a lot fast than the mag would index and taking and smashing or taking a bite out of the plastic fin while it was completing its turn. It seems like now the magazine has rotated further so the bolt tip is not catching the plastic fin in the magazine but catching the pellet in motion in the middle and pushing it into the breech (where the pellet stops) and not into the barrel. It seems like more spring pressure to speed up the magazine could help. 


The pics show pellets that where cut by the bolt probe tip, one stuck on the probe tip (that is what maybe leads to double cycling, pellet stuck on probe doesn’t fire, trigger pulled again cycles bolt with pellet stuck on tip and now two pellets misdeed and jam.) The pop-pop noise SOUNDS like double cycling but magazine indexing does not appear to happen again until the trigger is pulled again. 

 

[acmillerus]

 

[acmillerus]

It might not be clear from the pics the pellets have a cut on the dome from where they hit the breech instead of entering the barrel. Heavier pellets (18.1 JSB) tested with the Lane set up end up the same way.

It seems like the basic issue is slowing down the bolt cycling (or speeding up the magazine) to manage the increased reg pressure or increased airflow volume. It seems possible that expanding the plenum volume has a different impact on the bolt cycling speed than increased pressure does. It also seems possible that similar to the other TP repair there is something else going on here. The goal now is to get this system cycling reliably again to figure out what the issue is. So, it looks like turning the reg pressure down to where it cycles reliably is next. Then ensuring there is nothing really strange happening I have been missing, and then trying to pressure up again. 


In the beginning I did not want to mess with the bolt spring, but perhaps a lighter/shorter spring to slow down the bolt return without smashing the forward assist button if I can find something that fits maybe? It seems like I can print those here so smashing them is not the end of the world. Maybe printing one with soft flexible plastic like TPU would be helpful. Any suggestions appreciated. 


Also, appreciate the suggestion about considering other rifles, but messing with the SAM to understand and hopefully improve the system is part of the game. Ideally the process would be a simple recipe when it is worked out too, but that might be a stretch at this point. 

 

[FuzzyGrub]

The links below will show very similar looking pellets from my 1st SAM. Unfortunatly, do not know what caused it. Gun went back to PA, and got lost somewhere on its way to John at Crosman.

index.php (640×480) (gatewaytoairguns.org)

https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=180662.0;attach=338255;image



Getting the "bop-bop" on the first shot, after the T-bolt is slammed shut, is interesting.

Those 10 single shots look consistant. Did all of them make the "bop-bop" noise? If so, try again with the load assist button pushed in. Did the noise or velocity change?

Are the mags still fitting tight to the barrel? Snapping in? If not, they are moving during the cycling. Also, keep an eye out for cracks eminating from the gold pin in the clear cover.

I found it helpful to label the mags to track problems.

The hammer not latching on lower spring pressure is bugging me. O-ring still on the back of the valve?

If you are still on the original barrel O-ring, it is over due for change. A semi-auto is much more abusive to this O-ring than a manually closed bolt. Get a bag of -009 O-rings. Replace and silicone grease the ID.

At the 75% larger plenum, is that with the smaller one Lane provided?

 

[acmillerus]

Hey thanks! The pellets ending up like that can’t be a coincidence… 



All the single shots did do the double bop at 2050 psi I’ll try again as it is back together (will test at 1650 and see if it will cycle normally again.) if it is still bopping I’ll try the assist and let you know. 


The mags generally fit well, I have one that is getting a little tired as the first test and a more like new mag to follow up. These are labeled to keep that somewhat controlled. I was worried a bad mag could throw off all the info when I should have been worried about the TP… 

Will double check on the valve ORing but it was on when that test was done I think. That surprised me too, I could eliminate the bob-bob with less hammer spring but then the hammer stopped latching. I’ll try that one again too. 


I’ll replace the barrel Oring and post if that changes anything.

the ballpark 75% larger plenum is with the smaller lane reg body, the plugged gauge block, and a couple bushings to space the Lane gage port just a hair under center on the main tube. Close enough to center the gauge does not contact anything. nice to have the double check on the regulated pressure. I have been leaving the air supply and gauge on the filler so I could monitor the pressure in the tank. It doesn’t seem like that would change anything but let me know if that could be a problem.

Will report back a couple more tests later today. Never had one of the magazines apart yet so it might be time to see if there is an easy way to increase the spring tension in there? I think there is a thread somewhere about standard marauder magazine conversion for the SAM to reference. 


Thanks again!

 

[FuzzyGrub]

No "bop-bop" on the single loads, but got them on the first shot of the mag. Hmmm.



There are two little "nubs" where the mag snaps on the barrel. When they wear down, the mag starts moving side to side. I slow motioned filmed that when I was trying to get Swede large capacity mags to work.

 

[acmillerus]

Sorry I was unclear, the single shots did do the bop-bop thing. 


I’ll look at the mags more closely and see how they are in the breech. I have been using 5 magazines labeled 1-5 and generally in that order. Magazine 1 was hammered by the bolt probe beyond usability early on, but I carved out the fins and it came back to life, so I have been trying that one first to see if it will get beat up. Started watching the mag cycling after the shot to avoid double loading and jamming but didn’t notice anything odd. I’ll try to get some slomo iPhone video and see

 

[FuzzyGrub]

No, I mis-read it.



No need to put in a mag if it is doing the "bop-bop" on single shots.

I checked my SAM with the load assist pushed in and manually pulled the bolt. It does hit the back of the load assist. So, while that adds spring tension, it is also shortening the distance the bolt has to go. You can still test it on the single shots to see if velocity or noise changes. Don't do it with mag inserted.

 

[acmillerus]

So I went a little off script here to do another experiment.


The barrel gasket was being eaten by the bolt probe around the air inlet. Maybe 1/2 of the ORing was removed. Replaced with a new ORing and took. Close look at the bolt probe, it was sharp around the air inlet where it was opened up. I took a mini harbor freight file and smoothed it out. I wanted to know if the bolt would cycle slower (to give the mag time to complete indexing.) I removed the load assist button and the rear buffer spring and a rubber washer was put in their place. This added about 1cm to o the spring and reduced the tension on it. I was able to probe the bolt each shot and see if there was a mis-feed, although the sound of each shot was also a clear indicator. The bolt was probed before each shot and would still seat completely by letting go of the T handle from full back and letting the slightly less tense spring seat a pellet. The “chopping” pellets moved from the top or left side of the probe (as in the picture above) to the bottom of the probe. This was very consistent, I would manually cycle after e very shot and every other pellet would be stuck on the probe. Except at the end of my beater magazine, the last theee shots cycled and fed correctly and all fired at exactly 907 fps. I loaded a better condition magazine and the pellet chopping resumed in exactly the same place on the probe. I wanted to carve up and drill out a big saltwater sinker to add mass to the bolt and slow it down further with the lighter spring, but that does not seem like a generally reliable modification. Adding mass to the bolt might have a positive impact on how much total power or how heavy a slug the rifle will fire, but I’m not quite ready to do that. 


Anyway, less bolt spring tension allows the magazine more time to cycle but it seems the dome is still getting caught on the magazine exit hole or the entrance to the barrel. Maybe opening the exit on me beater magazine is worth trying too. One of the pel

lets popped off the probe and stayed in the magazine, this shows how the bite of the probe has shifted. 


The reg pressure was not changed since the last tests, and is reading about 135 Bar. Fortunately the Lane regulators are precision adjustable, the 1/8th turn marks are 5 bar and consistent with the gauge reading. I’m going to try turning down the hammer spring and then drop the reg pressure and see what happens. I’ll leave the forward assist out for step one then put it back if I get to normal cycling with less HS or reg pressure.

UPDATE:

The shot string from this test is below. Consistent fail to feed every shot until the FPS hits 905, then 3 working cycles, then return to misdeeds.these were the last four shots in the most beat up magazine I have. 

Shot count: 25
Low: 821
Hi: 909
Avg: 884
Spread: 88
STD Dev: 21.3
821
891
891
879
900
898
830
884
881
891
891
881
877
898
884
905
907
907
909
884
895
877
868
879
861

 

[acmillerus]

Another quick update. Decreasing the HS by 2 full turns returned to no hammer rebound latch and low 650ish FPS. Increasing by 1 full turn of HS results in brief full auto burst but jam happens almost immediately. 850 FPS. 


The forward assist system is going back to stock and will try reduced reg pressure by 10 bar. However that works it seems like a return to the stock plenum size was shooting and feeding in a more predictable way. If the return to stock is still not cycling correctly then there is a gremlin…

 

[acmillerus]

Last update for today. Turned the reg down to 120bar, 1740 PSI. Fed some single pellets and got a surprising shot string:

Shot count: 8
Low: 882
Hi: 890
Avg: 889
Spread: 88
STD Dev: 4.1
895
888
889
893
884
890
882
891

every shot sounded normal so I loaded up three different magazines. First shot first mag brapped and jammed. 886 FPS. Cleared and tried again. Same thing. Tried the next mag, same thing. Went back and tried hand loading singles, sounds good, single cycle. Shot a few more singles no problems. Tried the third mag and brapp and jam. So…

The hammer spring was turned up a half turn (to max) during the last experiment trying to get a clean cycle and feed. When I had the thing apart to reduce the reg pressure and clear a jam I had to get pellets out of the magazine (it was stuck) and out of the channel in the middle of the THandle. I realized I had never taken the barrel off, saw the bolt probe and the barrel had a tiny almost unnoticeable misalignment, Loosedned the barrel, tried to adjust a tiny bit, tightened it up. And that is it. It is shooting 890 at 1740 PSI but will not feed from any magazine I have. Any thoughts on what to test or just go back to the more stock setup and see if it still cycles at 2050 like it did prior to changing the reg and plenum size? 

 

[acmillerus]

From the last test. Reg pressure, and pics of typical misfeeds

 

[acmillerus]

Hi, and apologies for the delayed wrap up. First I want to thank John (FuzzyGrub) for all the help and patience. We are lucky to have you helping us here John!

The short version of a long story is, the bolt cycling system, and bolt timing coordinated with the magazine indexing speed, is sensitive. Not impossible to mod at all, just a seriously delicate balancing act. Slowing the bolt down with more airflow is the primary challenge. I did this with ORings on the spring end of the bolt that rub a little as it cycles. The bolt Orings can be added or subtracted to tune to a specific power level (reg setting, hammer spring adjustment, and pellet type). Eventually I got to a reliable feeding 860 FPS with JSB 18.2 domes, or 960 FPS with Crosman 14.3 domes. The other part of this I did not anticipate was finding a Barrel Oring system that would be stiff enough to not blow out every shot and soft enough to let the bolt slide freely enough to cycle completely. Ill post data and shot strings. All the earlier stuff is in this thread so Ill dig for and post a final before and after. I am getting happy with the end result, but it is unlikely I would go down the rabbit hole if I knew what I know now. However, I do like messing with these things so no loss at all, just more balancing than anticipated. Basically what John said at the very beginning... 

 

[FuzzyGrub]

I thought it was going to be an issue to deal with, but not as finicky as it turned out to be. You left no stone unturned in trying to balance it out, though. Enjoyed our conversations.

 

[acmillerus]

Hey John thanks again!

Here is the final with Crosman domes 14.3 and JSB 18.1. The FX Chrony I use dropped a couple shots on the JSBs. 

One last note, I ended up polishing the end of the bolt that goes into the barrel and this seemed to get better results. 

 

[Airgun-hobbyist]

Hey John thanks again!

Here is the final with Crosman domes 14.3 and JSB 18.1. The FX Chrony I use dropped a couple shots on the JSBs. 

One last note, I ended up polishing the end of the bolt that goes into the barrel and this seemed to get better results. 

@acmillerus That testing and tuning you did payed off well! With your additional porting, the 14.3 grain domes are averaging 20 fps faster than my setup at 940 fps. Try the JSB Hades 15.89 grain pellets in there. You will probably be pushing them at a near perfect 930 fps with no jams or loading issues. They produce really consistent and tight shot strings too.