Having problems with SBR 12.5 with suppressor

[chase45]

Okay a little backgroud. This gun has ran perfect through classes and alot of abuse. I finally got to pick up my M4-2000 a few weeks ago. Since then this SBR has had some problems

I believe its over gassed. I was getting bolt of base FTF and failure to lock. This is with an H2 buffer.

I ordered a new spring and H3 buffer thinking this would be enough to slow it down, nope still habing problems

Specs on the gun is
12.5 WOA. I dont know how many rounds to be exact. 6-8k at least
Carbine gas system
Now a h3 buffer
Semi auto carrier
ammo is generally m193 clone reloads however the test below was done with .55 gr hornady ammo

I really dont know what else to do. The rifle now is generally reliable with the H3, but does not lock open every time

Also the rifle seems to only malfunction when it is clean, when it is dirty it will lock open and have zero malfunctions

this is the test I did today with factory ammo and different buffers

Okay, I ran some factory .223 hornady 55 grains through her just a min ago

Carbine- fail to lock on pmag- 1 oclock ejection

h2- fail to lock on pmag- 2oclock ejection

h3- locked a little over half the time ejection was just a little better than 2 oclock

I did have no jams this time. I cleaned the gun before shooting, As I mentioned earlier it seems to do worse when it is clean than when it is dirty. So based on all this and the fact it runs better dirty than clean I must assume that it is overgassed

I guess I will order a 9mm buffer and a auto carrier and hope that will be enough to slow down the bolt

I really would like to stay away from an adjustable gas block.

any advice is welcome

[csmith]

Seems simple, but did you check your bolt catch to ensure it's not overly worn? Could be a small thing to overlook which would make these seperate issues. I'm no help with the other problems.

[Stickman]

Consider the Vltor A5 buffer system, it may be what you need.

[Eric]

Does it lock back with an empty mag in place, manually pulling the charging handle to the rear? Am I understanding correctly that it runs okay without the suppressor installed?

If an H3 with a new spring didn't do the trick while running the suppressor, you might be stuck going to a heavier action spring, like the Wolff #16501. It's not that uncommon to have a carbine that runs fine otherwise, but chokes when the suppressor is installed and the dynamics of the gas system are changed. Obviously the suppressor is creating a condition where the back-pressure is greatly increased.

[markm]

Definitely sounds like excessive bolt speed. The Vltor A5 has a heavier buffer option. Although the standard A5 buffer might do the trick.

Another thing I've dealt with in similar circumstances is the Bolt catch plunger spring. I've found more than couple that were way too strong. I've gotten to the point where I'll check that out before I assemble the bolt catch on a build... and I cut the spring down if it's too strong.

Be careful with those 9mm buffers... some don't have floating weights and can cause bolt bounce.

[tpelle]

I just read somewhere that the gas port pressure of a 20" rifle is 13.6KPSI, while the gas port pressure of an actual military M4 is 26KPSI. What ammo are you shooting?

I gather that military M4 rifles worked OK with M193 and M855 ammo, but they are experiencing a spike in bolt-over-base jams, extractors jumping or ripping through rims, and bolts breaking with the newest "green" M855A1 high velocity ammo that the Army is trying. Perhaps they've reached the limits of the envelope with the M4?

[markm]

I just read somewhere that the gas port pressure of a 20" rifle is 13.6KPSI, while the gas port pressure of an actual military M4 is 26KPSI. What ammo are you shooting?

He already said that he's shooting an M193 clone.

I gather that military M4 rifles worked OK with M193 and M855 ammo, but they are experiencing a spike in bolt-over-base jams, extractors jumping or ripping through rims, and bolts breaking with the newest "green" M855A1 high velocity ammo that the Army is trying. Perhaps they've reached the limits of the envelope with the M4?

That sounds like an ammo issue. The M4 has functioned fine for years. I've not read any accounts of these issues... but if it's suddenly a problem, it's not hard to see that the ammo is to blame.

[chase45]

Little update

No the bolt catch is fine locks back every time without suppressor, It will lock back 3/4 the time when the rifle is dirty and the h3 buffer, This is the first gun I have ever owned that works more reliable stupid filthy than it does clean.

Ive ordered a heavier spring, Sprinco blue

Also ordered a heavier carrier, BCM Auto.

Ill report back with my findings from that.

On the A5 stock, I really enjoy the UBR stock that is on the rifle right now. I will replace a barrel before I order a new stock. The barrel port size is the problem anyways.

If the spring and carrier doesnt solve the problem I have someone who has offered to send me an X buffer to try out.

IF that does not solve it Im tossing the barrel. It has had probably 6-8k through it, maybe more, I dont keep count.

But heres some gun porn to keep this thread interesting

Rilfe in question

[markm]

I'd ditch the barrel then. Just my $0.02, but any rifle that needs those goofball rainbow bright springs to run is a no go for me, daddio!

[chase45]

I'd ditch the barrel then. Just my $0.02, but any rifle that needs those goofball rainbow bright springs to run is a no go for me, daddio!

I'm with ya man. I'm not real fond of the idea either. But 20 is way cheaper than 350.

Ill be going noveske next time around. And I'm going to have the barrel dimpled. Like they did on the kac pdw. Just trying to squeeze another 1k or 2 through it before then.

[TehLlama]

As an avid UBR fan, it still may be worth taking a harder look at running an ACS-L and the A5, at least with that upper; but all told, if you're already looking at running a dimpled Noveske barrel I don't think you'll be dissuaded from that. It's what I'm looking at doing, but for a 16".

[chase45]

As an avid UBR fan, it still may be worth taking a harder look at running an ACS-L and the A5, at least with that upper; but all told, if you're already looking at running a dimpled Noveske barrel I don't think you'll be dissuaded from that. It's what I'm looking at doing, but for a 16".

Afraid to change it, balance is perfect right now

[markm]

I hear you. A balances weapon feels lighter than it is, and is nice to shoot.

[chase45]

I got an auto carrier and the sprinco blue spring in yesterday.

After I installed them I shot about 30 rounds. For the most part I only loaded a couple in several mags to see if I could get any issues to crop up.

No jams, and she locked open every time. I will shoot more this weekend, because 30 rounds really isnt a good number to see if its working correctly yet.

The ejection pattern still indicates the rifle is over gassed. Its shooting casings about at 230 area. However they are not going as far distance wise from the shooter. I dont mind the rifle being slightly overgassed as long as malfunctions are not occuring. Only time will tell I suppose if this has helped the issue.

Oh meant to add, I dont know if its the weather or if this parts slowed down the bolt more or what, but it seems a tid bit quieter to the shooter now. But that could have just been the weather on that day