Suppressors First Round pop & accuracy

[ShrockWorks]

I was testing some loads and shooting 5 round groups last weekend. As I fired the first shot of each series, I noticed that on the first round, the brass would come out MUCH cleaner and the primer flatter than the subsequent four rounds.

My theory is that since the gun sat with the bolt locked back b/t 5 rnd strings, the can had a chance to refill with oxy and some unburnt powder was was burning in the can and effecting the gas pressure and timing of the action.

This was very repeatable and very obvious.

I'm not sure the two are related, but I wasnt very happy with my groups. about 85% of the groups had one flier that was low of the main group.

I need to do some more testing and see if the first shot was the one going low, but can anyone shed any light on what I was seeing and whether or not it is a concern with regards to accuracy?

AAC-7.62 SD w/ Blackout 51T Muzzle Brake
.308 AR Mega build
16" Rainier select barrel
Fulton Carrier
JP Bolt

 

[RollingThunder51]

Re: First Round pop & accuracy

Let me see if I have this right...

You are shooting five (5) round strings. You are shooting them slowly.

After each string:

1. The bolt is locked back, opening the chamber, the can remains on.
2. After each string you are pausing for a number of minutes.


When firing your strings:

1. The first round point of impact shift is uniformly lower. You feel that POI shift is not appropriate.

2. The brass of that first round is cleaner.

3. The primer of that first round shows more pressure.

4. The remaining four rounds have less POI shift and they are grouped to a degree that you feel is appropriate. Brass is dirty, primers appear normal.

Have I got that right?

 

[ShrockWorks]

Re: First Round pop & accuracy

#1 I'm not sure about at all. I had one low shot in each group but I'm NOT sure it was the first one. The flier and the brass difference on the first round may or may not be related.

#2&3 - Yes

Everything you posted other than me being sure that round #1 was the flier is correct.

The brass was definitely cleaner and the primer flatter on the first round. That may or may/not have any relation to the one low flier, but something different was obviously happening on that first shot based on the brass condition.

I'm just wanting to understand what was happening with the brass on the first shot and what implications it might have on accuracy...if any.

 

[HPLLC]

Re: First Round pop & accuracy

With an autoloading AR, that will be harder to diagnose. If there is any play in the upper to lower fit, or if there is some play with a collapsible stock, that can contribute to fliers (elevation related).

I noticed when shooting my LMT MWS that the stock had some up down play at the heel of the buttplate where I use my rear bag, so I used a couple layers of clear tape to remove some of that. Still loading the gun the same way every time is going to help keep the groups small in either event, because even small residual play will have some effect.

 

[RollingThunder51]

Re: First Round pop & accuracy

In truth, you have some work to do. We all come across oddities with our suppressed rigs, Griff's example is perfect. But, to help you, you have to do your part.

Take the time to shoot a solid series without your can. Not just two sets of five, but more like 5 or 6 sets. All doing your part to insure that the best that can be had without the can is shown on those targets. Shoot a round, mark your sheet as to where that round went.

Then add your can and do the same. Shoot it, record it. Pause between your strings if that is what you think is making a difference.

Then one last thing. Get a clean target and shoot 5 without your can, mark every hole with a pen. Then shoot five with your can, mark them. Rest with the bolt open, and then another marked string. Post that target and let us know if you still have an issue.

Does it look like this?

 

[ShrockWorks]

Re: First Round pop & accuracy

That for the advise guys and I agree I have a lot more more to do. This was the first time I ever shot this build, so I dont really know what's normal or what it is capable of yet. I was also testing different loads in each 5 shot group so too many things were changing. I did manage to two very nice groups out of 3 5 shot with the barnes 130gr ttsx. It was more like 2 out of 15 with the other 3 loads I tested. It was a very windy/gusty day so extra L/R dispersion was expected, but it was always vertical that was blowing groups.

There IS what seems like excessive vertical play in my ACS stock. I put one piece of electrical tape on it before I went out and it helped, but it didn't fix it entirely.

I'll work with the stock, shoot some groups w/o the can, try both with the same load and note each shot. I usually only shoot handloads, but I ordered some Black Hill Match ammo to use as a control too.