Marauder Semi-auto power tune mods

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Got it, thanks! So far so good, lost a little pressure drop overnight but shoots and cycles like new. Will start getting some shot strings to look for the reg drop off after work if there’s any light left today. The pics of the gauge are about 18 hours apart. Let me know if the leakage will compromise the test? 
EE4680DF-06A1-4B86-9F6D-9837EA9875D3.1640909289.jpeg
B811BD58-2FDA-4847-AB57-F4C7E3DF2981.1640909291.jpeg

 
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Picked up some Crosman domes when I saw some locally and used the 14.3gr for this testing. The previous numbers were generated with pellets from a ziplock bag and I may have been mistaken about their identity. Possibly Benjamin 14.3 (same as crosman I assume) or maybe JSB Jumbo and not the jumbo heavy as I thought. Anyway here is some real data from the 14.3 crosman pellets. The pressure gauge is not ideal to read accurately, so I took the first 16 shots from each string so all are very likely regulated. Let me know how to do this if I missed the bus the way I collected the information please?

A quick and dirty ballpark summary:

861 FPS average stock vs 868 FPS average with the forward assist button pushed

Zero feeding issues 

FPS drops a bit over 2000 psi, so ballpark 2250 seems reasonable for the stock reg setting. 

I was hoping to see a larger increase in FPS with the forward assist pushed in, but there is a change. The zero mis feeding situation seems to say more spring force and more pressure might be ok. I was using the button I made that was a little longer than stock. This was done pressing it in with my thumb while shooting as the extra length when it was latched into the housing would not allow full magazine removal or full bolt cycling. Let me know what you think here before I drill out the port in the valve, port, and breech please? Also, any recommendation on a replacement pressure gauge? There is a chance the gauge will end up showing regulated pressure so higher resolution in a lower pressure range might be better while dialing this in.

All the data was generated using the FX chrony, no missed shots or outrageous outliers obvious to me, but I am an admitted noob here to all this. Any suggestions are appreciated. 

All data:

Stock configuration from approximately 2750 psi down, string 1:
863
865
856
875
858
849
858
875
875
849
849
849
847
875
854
858
877
858
856
856
844
858
872
872
858
854
842
865
835 @2000PSI
837
844
854
854
835
823
847
849
826
837
842
817
835
807
812
828
807
803
821
817
793


Stock string 2 Fill to about 2600 psi:
861
861
860
861
861
863
861
861
879
860
863
858
856
875
858
868
851
849
847
863
856
858
854
835 @ 2000 psi
849
821
830
819
844
823
837
817
833
810
823
812
803
800
789
812 

String one, forward assist pushed:
868
863
854
886
865
863
870
863
870
865
883
854
863
884
870
854
868
856
861
879
856
863
854
877
844 @2000 psi 
854
854
851
837
835

Forward assist in string 2:
865
851
884
861
863
861
879
861
861
881
886
863
875
863
881
863
854
881
863
881
884
861
884
851
881
861
856 @2000 psi
861
856
849
849

 
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That slow leak will not impact testing.

That gauge does suck with 500psi increments. Going to check mine, but petty sure it is 200psi

Crosman sets the reg to 2050psi.

Small increase with load assist with the lighter 14g pellets. Try the same test with 18g JSB and then the 25g ones. The 25g is where I'm hoping to see more of a difference.



PS: Going to have limited access through the weekend. Didn't want you to think I left you hanging. Tuesday should be back to normal. ;)


 
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Thanks for the heads up! Sounds like a bit over 2000 psi for the reg was not too far off. The 2050 number is good too, there is still a little room to turn it up. It was surprising to me that even shooting pretty fast the Crosman regulator and small plenum seemed to keep up. 


I’lll test the heavier pellets in stock form. It makes perfect sense that the heavier pellets would provide more resistance and push the bolt open relatively earlier. So heavier pellets with higher pressure and/or more air from a larger transfer port/plenum would probably push the bolt with the stock spring even faster. 


Is it reasonable to open up the transfer port after getting the stock numbers with heavier pellets with and without the assist pushed? It seems like that would be the next step regardless of how the heavier pellets work with added bolt spring force. If heavier pellets show increased gains, then more or higher pressure air would proportionally move in the same direction right? Let me know if you think I should hold off on the irreversible drilling transfer port part of the surgery until we have a little time with the numbers from the tests please? 


Another little benefit of doing this testing is getting familiar with the SAM trigger. I can probably get that dialed in along the way too. I’ll post the info when I have it. Enjoy the weekend and merry new year!
 
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Yes, after testing the heavier pellets, can go to the TP. Take it from 0.140" to 0.160". The easiest way I have found, is to incrementally go through number drills to the size needed. Going in one step you can easily get too close to an edge, and be too thin and can collapse. With the small increments, the drill wants to follow the existing hole. Harbor Freight has number drill sets very reasonable and work fine for many airgun uses. I use calipers to check the drill diameter, for the final size. Stock TP are easy to come by if needed to return to stock.

After TP change, verify power and cycling with the crosman 14.3g, assist in normal out. Then do the same tests you did with the assist pushed in, for 14.3, 18, and 25 gr pellets.

If you could, wait for my better availability before changing the reg pressure. On the next fill, try to calibrate the 2K, 2.5K, and 3K gun gauge marks to your fill gauge, which is probably more accurate. .
 
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Based on your last recommendation I have been using the fill gauge on the cheap pump I have. When the pump gauge reads exactly 3000 psi the marauder gauge is a little over 3000 psi. For the last set of tests the marauder was filled to exactly 3000 psi before every magazine to try to normalize the pressure variable. 

You were exactly right that the heavier 18 gr pellets showed improvement over the lighter crosman domes with the assist. 


One thing you might understand that did not make sense to me: the rifle cycled without fail until getting to 25 gr pellets. With the 25gr, the first magazine had 1 failure cycle. Looking at the magazine after it had clear dents in it from the bolt. To the point it was harder to load pellets. I changed mags to be safe for the assist on test. 

I was thinking with more back pressure from a heavier pellet the bolt would blow back faster and change coordinated timing of the magazine and bolt spring. So it seems to follow that increased bolt spring pressure would help balance the increased blowback. But, the rifle would not cycle completely at all with the assist pushed in. It would fire the first pellet from a magazine then fail to reload. I repeated this enough times to chew up one magazine thoroughly, we were expecting this. 

Unless you have a different take on this it seems like the data from testing the 25gr just means they barely cycle with the stock set up. It seems the magazine spring is turning the mag too slowly for the increased blowback and bolt return spring force with the assist pushed in. Like the magazine cycle rate is going to be limiting unless there is a way to get more “dwell” time with the bolt. Do we need increased air pressure from the valve with heavier pellets and more bolt spring? I did not want to drill anything since I am still unclear about how to fix the current cycling issues. 

I can get better pellet speed data from the 25s with the forward assist but will need to single load…

Let me know your thoughts please? 



Data below: 

25 gr stock

Shot count: 9
Low: 666
Hi: 701
Avg: 682
Spread: 35
STD Dev: 11.6
701
684
670
684
673
680
687
666
696

25 gr stock with assist

Shot count: 7
Low: 677
Hi: 691
Avg: 686
Spread: 14
STD Dev: 5.3
682
689
684
689
691
677
691



18gr stock

Shot count: 10
Low: 784
Hi: 814
Avg: 797
Spread: 30
STD Dev: 9.1
784
784
798
814
796
798
791
800
800
805

18gr stock with assist

Shot count: 10
Low: 789
Hi: 819
Avg: 806
Spread: 30
STD Dev: 10.9
798
791
817
819
789
814
800
814
812
803
 
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Just for clarity, did the 25g JSB cycle fine without the load assist pressed in? Or just worse with the load assist?

No jams? Just cycling errors? Did that first shot sound loud or off?

I assume the bolt is hitting the revolver and on the edge of the pellet tails.



The numbers with and without for the 18.1 and 25g are essentially the same. That slight difference could just be just normal variation.

You can index the mag spring to have more rotational speed, but wouldn't do that at this time.

When you refill the gun, right after the check valve opens, check the low pressure compare, pump to gun. Also try to put a value to the amount over on the 3K fill.




 
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Just to be clear, I got the most gains in fps from adjusting all available spring pressure and opening up the transfer ports in the probe, the small barrel transfer port, and the valve housing. Because the small barrel transfer port could not be opened up safer than 0.156" that is where I stopped step drilling. With a new bigger barrel transfer port and bigger i.d. sealing o-rings, everything can be opened up even bigger. The limitation to airflow would then be at the pellet probe.

After drilling the valve body port, I also went inside and radiused the corners and opened the space up in there for more volume (using a Dremel tool and small cutter bit). When drilling and grinding in the valve body, think of that as the first area that the high pressure air sees when the valve first gets knocked open. Remove metal accordingly so the air flows most efficiently up and around the corners and out the port, rather than leaving 90° drilled corners in there.

IMG_20210326_1905304424.1641246150.jpg

 
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Ok thanks, will do. The assist spring force does have some effect, but maybe more of a balancing challenge with the magazine than I appreciated. My goal is to keep the reliable semi auto mechanics even if that means lower power. The more I look at the semi auto mechanism the more I appreciate the design and what it is. The AEA Terminator has the funniest functional (and even efficient) mechanism I ever saw but with some significant trade-offs. 

Back on task, Ill get the port, breech, and valve drilled out and cleaned up today and should have more numbers from the 14 and 18 gr pellets sometime tomorrow. I ordered an extra valve and transfer port from Crosman so it would be possible to put back pretty close to stock if needed.
 
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Ok thanks, will do. The assist spring force does have some effect, but maybe more of a balancing challenge with the magazine than I appreciated. My goal is to keep the reliable semi auto mechanics even if that means lower power. The more I look at the semi auto mechanism the more I appreciate the design and what it is. The AEA Terminator has the funniest functional (and even efficient) mechanism I ever saw but with some significant trade-offs. 

Back on task, Ill get the port, breech, and valve drilled out and cleaned up today and should have more numbers from the 14 and 18 gr pellets sometime tomorrow. I ordered an extra valve and transfer port from Crosman so it would be possible to put back pretty close to stock if needed.
Click to expand...

I forgot to add, I did give it a few hundred more psi of regulator pressure (a quarter of a turn). The only drawback of any of the mods was a slightly lower shot count after adding more reg pressure. But I was looking for more power versus shot count anyway. And, I have not had a misfeed either. So you shouldn't have to go back to stock.
 
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I drilled out the port, valve, and breech. I’m going to call this phase 1 tps mod, because I did not do any more work on the valve and port than drilling the port hole and removing burrs. I believe the hammer spring is going back to stock each time (about 8.5 turns in from all the way out low spring tension lock) but this could be a variable also. The valve seems like it would handle being drilled out significantly in addition to the port mods the OP did. I ruined a set of heads to a small block chevy when I was a kid as my first attempt at “porting”, so I’m trying to proceed slowly here. I have pictures what was done so far if anyone is interested. 


Does anyone know if increases the valve volume specifically has a different impact than total plenum volume including valve? 



Seriously appreciate the note about not having to go back, the TPS mod (even with the assist pushed) resulted in 0 misfires! I am looking more for heavier pellet speed and stability more than shot count also. 

Here is confirmation of the OP that port mod is really effective even done very basically. I’ll repeat this with a 1/4 or 1/2 turn reg increase and see if that adds anything additional. The std dev changes after the port mod was another surprise. 

Any thoughts on hammer adjustment before or after the regulator? 

Here is the data using the 14 and 18 grain pellets:

14 Gr TPS Mod

Shot count: 10
Low: 921
Hi: 937
Avg: 926
Spread: 16
STD Dev: 4.5
937
928
928
928
926
923
926
923
923
921

14 Gr TPS mod with assist

Shot count: 10
Low: 919
Hi: 928
Avg: 923
Spread: 9
STD Dev: 2.8
928
919
926
921
926
923
921
923
923
921

18 gr TP drilled

Shot count: 10
Low: 849
Hi: 858
Avg: 855
Spread: 9
STD Dev: 2.4
854
858
854
856
856
856
856
854
854
849

18 gr TP mod with assist

Shot count: 10
Low: 847
Hi: 856
Avg: 850
Spread: 9
STD Dev: 3.0
851
854
851
849
856
847
851
847
847
851





 
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Good gains there with almost 30fpe ave with the JSB. :)



It sounds like you modified more than TP. I assume it was valve exhaust, TP, and barrel port. Do you have the before/after measurements?

Unless there is a SAM specific poppet, the valve is probably the same as the standard Mrod valve in relation to valve seat, pocket, and exhaust locations. On the standard Mrod valve, 0.187" exhaust is about as far as you can go before hitting the seat and ruining the valve. Above 0.187", you have to angle or use oblong porting, which is not that easy DIY with caveman tools.

Before opening the valve even further, get the inside measurement of the flow through bolt and the inlet to it. That is going to be the bottle neck, and you don't want to go much more than that, with the upstream porting. 

Your current hammer spring setting seems high at 8.5 turns CW, with a stock setting reg. For regulated guns, the HS is really just to balance out what the reg is set to. Mainly to get a flat fps through the range. Here is an article on the G2 Fortitude and how HS effects the curve: Fortitude HS adjustment

Myself, have not spent allot of time rounding and smoothing edges. I haven't seen much gains when I have, either. To me, this is not laminer flow, it is just a very short duration (milliseconds) blast of pressure. On the other hand, it doesn't hurt to do it. ;)




 
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Sorry about that I misunderstood… unfortunately being an amateur the TP, the valve exhaust, and the barrel port all parts of the same “transfer port” system. So I might have jumped into the deep end sooner than intended! I do have a stock transfer port and stock valve so it would be possible to go back closer to stock to get measurements, but there does not seem to be an overwhelming reason to go back. 

Worth specifying I did not remove the barrel or bolt. So drilling into the barrel port the bolt was held out of the way. After opening up the barrel port and seating the bolt/probe to line up the ports, I used the same drill bit to open up the bolt probe to the same size as the barrel port. This part was kind of a delicate surgery as a mistake would mean major surgery to replace the bolt/probe. The probe is thin steel and the rest of the passage is all soft aluminum. Worth mention I am a hack and limited to harbor freight caveman tools. Not the nicer junk there either… one drill bit was used for all port passage opening, it was a HF 5/32 that measured at 0.155 inches. The TP and valve exhaust both measured about 0.158 to 0.160, so the bolt probe is probably similar. This size seemed as large as the bolt probe port would get without really thin spots or making it out of round with the barrel port. So as of now the whole air path from the valve to the probe is about the same size. 


I’ll read up on the HS adjustment, thank you for the reference! 

So is the next item moving the reg for plenum size or increasing pressure with the same size plenum? Adjusting the reg is probably easier with the stock parts. 

 
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I just wanted a clear picture to what has been modded and to what.

I would not jump to the bridge/larger plenum, yet. Mark the reg/adjuster for baseline position, to return to. Turn the adjuster 1/4 turn CCW. This should be about 200 psi. Reloctite the adjuster.

Should shoot a complete string so we see where it is at, as far as HS adjustment. falling/rising/flat Knowing the new shot count and gross reg setting won't hurt either.



If at any point, you feel you have enough power, etc, no need to continue for my sake. It looks like reaching 30FPE (my goal) will be achievable. 
 
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Sorry about that I misunderstood… unfortunately being an amateur the TP, the valve exhaust, and the barrel port all parts of the same “transfer port” system. So I might have jumped into the deep end sooner than intended! I do have a stock transfer port and stock valve so it would be possible to go back closer to stock to get measurements, but there does not seem to be an overwhelming reason to go back. 

Worth specifying I did not remove the barrel or bolt. So drilling into the barrel port the bolt was held out of the way. After opening up the barrel port and seating the bolt/probe to line up the ports, I used the same drill bit to open up the bolt probe to the same size as the barrel port. This part was kind of a delicate surgery as a mistake would mean major surgery to replace the bolt/probe. The probe is thin steel and the rest of the passage is all soft aluminum. Worth mention I am a hack and limited to harbor freight caveman tools. Not the nicer junk there either… one drill bit was used for all port passage opening, it was a HF 5/32 that measured at 0.155 inches. The TP and valve exhaust both measured about 0.158 to 0.160, so the bolt probe is probably similar. This size seemed as large as the bolt probe port would get without really thin spots or making it out of round with the barrel port. So as of now the whole air path from the valve to the probe is about the same size. 


I’ll read up on the HS adjustment, thank you for the reference! 

So is the next item moving the reg for plenum size or increasing pressure with the same size plenum? Adjusting the reg is probably easier with the stock parts. 
Click to expand...

You did everything about perfect it seems, especially stopping short to needing major parts replacement. When porting anything, (just like with a motor) your goal is to make the air flow as quickly and efficiently out the poppet valve, up into the transfer port and then into the barrel. I look at the ports just as I do with engine's; get it flowing as fast and without any turbulence around corners as is possible. I concentrate on making the 90° transitions as smooth and efficient as possible, and it usually nets the most FPS gains. And just as with engines, big ports don't always mean fast airflow. There's a balance of port sizing and air speed. Given the limitations of the SAM's plenum space, the ports at .155" is probably about perfect for the semi-auto to function properly without chasing spring pressures and trying to get plenum volume perfect. Though, if you really wanted to get into removing metal for plenum volume, there is a lot of aluminum available for the removing on the valve and regulator body's (where the spacer sits) that would locate everything in it's respective places.
 
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Thanks for all the pointers in the right direction! Getting the right feedback undoubtedly saved me from blowing this thing up already. To satisfy my own curiosity I shot 1 string each of 14, 18, and 25 grain pellets with one full turn more of HS tension. I started with one more string of the least expensive 14gr pellets that was almost identical to the last 14gr string (STD DEV 3.5 and avg FPS 919). Short summary of increased HS tension is almost no change in STD DEV or FPS from the 14 and 18gr pellets. Two misfires with the 25gr, running 720 avg FPS and std dev of 3.3. So I am probably turned in more than stock was already. I returned the HS to where the was in all the previous tests, about 8.5 in from full out, and will keep it there for the reg pressure test next. 30 FPE seems like 1 drill bit away. I am curious if removing more material from the TP and Valve would do much given the bottleneck at the probe. However, turning up the reg a little is reversable. So that is whats next. Ill post up some complete strings with the current settings and then with the reg up 1/4 turn and adding more loctite... 

On question about the valve seat: I only have bad pictures of this, but I am assuming the seat is on the outside of the small space before the stem passes through the valve body where the exhaust port is? It looks like the port is set back in that space towards the stem guide part of the valve so potentially that part of the port could be pretty big? Ill post some pics to help explain
 
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