Umarex Notos Accuracy Troubleshooting

There's a good chance that I'm part of the 1% who got a notos that doesn't shoot laser beams, unfortunately I'm in no position to return the pellet rifle on account of incessant tinkering. So here's an account of my journey so far in the hopes that with some help I can figure out how to get the most out of this handy little hunter.

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All shots and groups are taken at 20 to 23 yards from bags. The pic rails are torqued to 15in/lb and loctited, the barrel set screw is 10in/lb and loctited, the scope rings to pic rail are 15in/lb and the rings to scope are 10in/lb. There is no wobble to things as far as I can tell. The regulator is at it's stock pressure, the hammerspring is 1 full rotation out from maximum tension, though previous adjustments seem to make little difference.

The notos has these mods: polished hammer, polished sear, 3D printed LCD extension, 3D printed folding AR style lower, 3D printed magazines from thingiverse, Bugbuster 3-12x AO scope, 3D printed handguard, barrel o-rings replaced with ones from aliexpress (linked here: https://www.aliexpress.us/item/3256...il_item.3.68d9f19cqsiqY0&gatewayAdapt=glo2usa in 10.2x0.8mm ) de-burred to try and keep barrel orings from getting chunked and a poor attempt at brass-rod crown polishing.

Pellets tested: Crosman Premier Hollow Points, GTO Predator Lead Free (best shooting), Polymag short (second best), H&N Barracuda Green, Crosman Red Flight Lead Free (horrible), H&N field Target Trophy Power Copper Plated.

It never got single hole groups, as seen here:

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At best I would get groups like this with GTO Lead Free:

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and the best group I got out of CPHP's were like this:

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So as y'all can see, nothing approaching the HAM review groups! Up until recently I'd been fine with the GTO's performance and it was accurate enough to hunt some squirrels with, but recently I've started to experience some left/right point of impact shift that I need to figure out. I've been getting more and more flyers and the groups have universally been opening up. CPHP's are getting as bad as 4" groups with flyers and the GTO's have widened as much as 1.5" with flyers from a clean barrel.

I mention clean barrel because it seems like the GTO's shoot better if I follow this procedure: clean barrel pulling cloth soaked with ballistol until no longer dirty, pull one clean dry cloth through, shoot ten rounds of lead pellets, shoot GTO's with improved groups. This procedure did not help the accuracy of the barracuda green pellets.

The barrel has seen an estimated 800 rounds of lead pellets and 600 rounds of GTO lead frees along with a handful of other lead free pellets. I also have a chronograph arriving tomorrow to help me troubleshoot and isolate what might be the cause.

And one final photo of two groups I shot just now, 5x CPHP in blue, 3x GTO in green.

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I think that's everything, I've tried to be thorough! Ty anyone who has read the whole post, apologies in advance if this is just a simple fix that I've overcomplicated.
 
Does the trigger (pull weight) vary by every couple of shots? If so, this is most likely due to a rotating hammerweight (which is held by the sear). This greatly affects accuracy in a (very!) detrimental way. Unfortunately I have no solution what can be done to fix (i.e. Stop) the rotation of the hammer weight.

I have a fix! Turn the sear face square, heat until a dull red glow, dunk in used motor oil.

I've seen three dead-soft hammers over three past 2 years, I had a replacement for the first (donated the one from my first Notos) and have case hardened the other two.

I can't say I recommend you do it because I have a forge I can heat without oblonging the hammer.

Once hardened, chuck it up in a lathe to check contricity and shape. I will polish and blue them when done.
 
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I have a fix! Turn the sear face square, heat until a dull red glow, dunk in used motor oil.

I've seen three dead-soft hammers over three past 2 years, I had a replacement for the first (donated the one from my first Notos) and have case hardened the other two.

I can't say I recommend you do it because I have a forge I can heat without oblonging the hammer.

Once hardened, chuck it up in a lathe to check contricity and shape. I will polish and blue them when done.

Heh, I think I'll wait to give that one a try.

At least until I can 3D print myself a lathe (seriously Chris Borge's looks kinda promising though).
 
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Quick follow up with some heavier pellets. Three shot groups, 20yd from bags:

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16gr-ish still doing the best, similar to some of the groups I've gotten with polymag shorts

18gr is ok, though a little inconsistent.

For those with experience, how much does your windage change between pellets? I'm having to adjust upwards of 10mil dots to keep things centered between the two JTS pellets.

I have the chronograph coming in later today to get a sense of velocity, after that it'll likely be new barrel time.
 
x2 on the advice to try some JSB/FX/Air Arms or JTS domes. I seldom if ever have gotten excellent groups with the ones you have been testing with.

And one other thing that popped into my mind as I was reading that the Crosmans and non-lead GTOs grouped best. Over the years working on barrels and dealing with various defects and workmanship issues, I noticed a subtle pattern that barrels with burrs and other poorly machined features tend to tolerate harder pellets better, like the Crosman pellets in particular. Not that they would group great but they would group at least as well as, or better than higher quality pellets. Whereas a meticulously smithed barrel capable of tiny groups would always achieve those groups at distance with quality pellets.

A long way of saying the barrel itself is still very much a suspect. If you’re interested, in my profile is a link to some popular troubleshooting topics. One is a barrel troubleshooting and accurizing guide that covers the most common issues.
 
x2 on the advice to try some JSB/FX/Air Arms or JTS domes. I seldom if ever have gotten excellent groups with the ones you have been testing with.

And one other thing that popped into my mind as I was reading that the Crosmans and non-lead GTOs grouped best. Over the years working on barrels and dealing with various defects and workmanship issues, I noticed a subtle pattern that barrels with burrs and other poorly machined features tend to tolerate harder pellets better, like the Crosman pellets in particular. Not that they would group great but they would group at least as well as, or better than higher quality pellets. Whereas a meticulously smithed barrel capable of tiny groups would always achieve those groups at distance with quality pellets.

A long way of saying the barrel itself is still very much a suspect. If you’re interested, in my profile is a link to some popular troubleshooting topics. One is a barrel troubleshooting and accurizing guide that covers the most common issues.
That observation about harder pellets tolerating poor machining is very helpful and definitely helps to explain why I might be grouping better with the non-lead GTOs despite their lighter weight. Thank you for that!

I got some JTS domes in yesterday and tested them out seen in post #10, they did ok but not spectacular. It seemed to me like within 5 shots there was always at least one flyer. Pellets floating around 16gr seemed to perform the best for lead pellets, both the polymags and the JTS 16.1gr domes having the best groups when I didn't have flyers.

I've decided to swap out the barrel with one from @Firewalker and will report back about any improvements once I've got it installed and tested!
 
Sorry, I just noticed this statement:
For those with experience, how much does your windage change between pellets? I'm having to adjust upwards of 10mil dots to keep things centered between the two JTS pellets.
A wildly changing lateral POI for different pellets has been a pretty consistent red flag for a defective barrel in my experience.
 
Sorry, I just noticed this statement:

A wildly changing lateral POI for different pellets has been a pretty consistent red flag for a defective barrel in my experience.

He has ran through the other possibilities, I think the barrel is the next point to check.

The proof will be in the pudding once I get his on return (I only do swaps) and see what went wrong...