Rem 700 SPS and JP recoil eliminator accuracy change

[dbgiayb]

I stumbled across Jerry's thread about issues with the removal of muzzle brake on his Rem 700 SPS while trying to solve a similar problem with mine. I had the barrel threaded on my Rem 700 SPS 20" 308. I removed the JP Recoil Eliminator from my DPMS gas gun, where it performed amazingly for back-on-target. I was looking for the same thing on the SPS. Up until that time, the SPS had MOA or better performance consistently. At the range, I was shocked to see that I couldn't get 3 MOA performance, even with match grade ammo. Everything was high-right spread in all directions in a 3-4 inch area at 100 yards. I shot at least 25 rounds in 5-shot groups. I couldn't believe it. After checking the scope for stability (no obvious problem), I removed the recoil eliminator and screwed the thread protector back on. Groups returned to 1 MOA and better. One thing that might be a factor is that the barrel may have been threaded too long. So I guess what I'm trying to figure is maybe the distance from the end of the muzzle to the eliminator is too little or too much? I'm not sure where the crown is supposed to be in relation to the muzzle device. I can find tons of material about screwing the things on but not much on where it's supposed to end up. Flush with the beginning of the device or farther?
Thanks, y'all, for any advice.
Tommy

 

[Scorch]

Barrel harmonics changed when you put the brake on the gun.

 

[dbgiayb]

700 accuracy and JP recoil eliminator addition

Thanks for the fast response, Scorch. You know that's exactly what i didn't want to hear and makes perfect sense. I'll just cap the barrel and move the JP back to the gas gun.
The 700's stock and barrel make it heavy enough to have low felt recoil anyway. I am fixin' to do a competition where you make 5 shots in 60 seconds at various distances and was looking for a few seconds edge.
I really appreciate it.

 

[Dixie Gunsmithing]

What Scorch mentions is spot on. When you can cut off or add to any barrel, you take a chance on changing the harmonics. In other words, by cutting off or adding to, you can hit the spot that will cause a barrel to vibrate. Barrel lengths are calculated to stop that from happening. They have a null zone, due to the design length, for off the shelf ammo. Even different cartridge loads can cause a barrel to set up harmonics, that regular ammo wont cause.

You might try clamping a weight to the barrel, say 2-5 ounces, and see if that helps. All you're doing by adding a weight, is tuning it, to a point that will not set up a vibration with that load. If it cures the problem, then you know where the problem lies.

 

[dbgiayb]

Thank you for the suggestion. I will certainly try it. And as a further bit of information, i could screw the recoil suppressor on until it bottoms, and still had a gap to muzzle mating. i used a crush washer first, then switched to a lock nut, but still have a gap of about 1/8". The barrel threading was performed by a non-gunsmith, but excellent machinist. Just by eyeballing it compared to my factory threaded gas gun, it just appears too long. That can't be good for harmonics either. Is it ok to use two crush washers or nut/crushwasher combo?
I know if the barrel gets cut back, then the crown will need to be duplicate also...

 

[tobnpr]

I agree with everything mentioned; it's unusual (brakes, like suppressors, will usually only affect POI, but not accuracy) but does happen.

You didn't mention if you handload- but if you do, I would (if it were my rifle) re-do load testing with either OCW or Ladder (or both) to see if I could find an accuracy node with the brake.

i could screw the recoil suppressor on until it bottoms, and still had a gap to muzzle mating. i used a crush washer first, then switched to a lock nut, but still have a gap of about 1/8".

On edit...

I should have read your post better. Please post a pic of this, as I'm not clear as to what you're referring to as a "gap". Did the machinist cut a thread relief groove? If so, the brake should spin past it and torque tightly- and SQUARE, to the shoulder on the barrel. I've installed at least a half-dozen of this particular brake without issues. It should- like any brake- torque to a shoulder.

Sounds like, for some reason, the brake is being held in place simply by hitting the end of the threads. A brake or suppressor (it's especially critical in the case of suppressors) needs a perfectly square shoulder where the tenon ends.

Also... you mentioned that he's an excellent machinist, but not a smith.
Is he aware of- and did he compensate for- the fact that the bore is NOT concentric to the barrel?? Question him about how he did the threading- if he just indicated off the outside of the barrel- and not the bore- this could also be your issue. Threads need to be cut perfectly concentric to the bore- if he did not do this, your brake could be "off center" of the bore by enough to cause an accuracy issue, but not enough to cause a clearance issue/brake strike.

Based on what you've described, your issue may lie with the way your muzzle was threaded...

 

[dbgiayb]

Thanks, tobnpr.

I did start the day with four test loads to try out, but after they failed miserably, I switched to 168 FGMM.
I think you hit it on the head, as far as the machining goes. I would almost swear that the barrel was NOT threaded concentric to the bore.
I'll fit it back up and take a picture or two. I had removed it and swapped it back to my DPMS 308 (which performs just fine). So, at this point, it looks like I won't be putting any muzzle devices on that barrel.

 

[dbgiayb]

tobnpr, I attached a picture I hope you can see. To say the least, I'm embarrassed that I didn't notice the fit issues when I put this on the first time. It worked so well, I didn't give it a second thought.
Here I have it screwed on and lined up with no crush washer or nut. There are about 3/8" exposed threads before it would even reach the muzzle crown.
I'm sure that would wreak all kinds of havoc when the bullet exits. Do you think there's anything to do here? Cut more threads and hope for the best? Cap the barrel and forget about it?
Thanks for any input.
Tommy

 

[tobnpr]

No problem I can see. When you said "gap"- I assumed you were referring to where the brake meets the shoulder.
I cannot tell from the pic, but the "shiny" part of the muzzle should not be rounded, needs to be flat and square to the bore.

The length of the thread tenon appears to be correct. JP recommends starting with the "standard" tenon length of .625 or so. From there, we screw the brake on and cut the shoulder further back until the brake times correctly (horizontally). The shape/design of the JP does not lend itself to putting it in the lathe and cutting it back to time it, so it needs to be done on the barrel shoulder.

Per JP:

" Best efficiency is obtained by allowing the muzzle to be recessed into the barrel nut by about a quarter of an inch. This has the effect of forming a cup that directs the expanding gas forward into the baffles.

So- no real issue there, it's never a "specific" measurement as it's going to vary depending on how much setback is needed on the shoulder.

Suggest back to square one- which is just asking the machinist whether he cut the threads concentric to the bore, or just indicated the outside of the barrel.

 

[dbgiayb]

Thanks again. i promise you won't have to educate me but once. I can probably reach the machinist this weekend and confirm barrel vs bore alignment. I missed JP's info, so that was all good. I attached a picture of that shooting session. The top left is 168 fgmm with the recoil eliminator mounted with a locknut. Bottom left is 168 fgmm with the recoil eliminator removed. All of the high-right groups were with the recoil eliminator on. 5-inch paper plate, 1-inch dot, 100 yd.