Sig Sauer M400 Enhanced AR-15 - Bolt Not Locking Back After Last Round Problem

Raff85 said:
I decided the bite the bullet so to speak and send the rifle to M.D. Charlton to have work done under warranty.... Shipping was a pretty penny, and I had to take off my ACOG so that will have to be re-zeroed, but I figured maybe it was the best way to address the issue due to the number of suggestions and possible complexities that are surrounding this issue.

Murphy's law will probably prevail and they won't find anything wrong with it, or the problem will be the cheapest part possible....
Click to expand...

The zero on your acog likely won't be off by that much.
 
guy has 750 rounds through the gun....

try again




Trinimon said:
I can think of a few things that may cause this:

- Mag catch location milled into the lower receiver is on the low tolerance side but high enough for the bolt to strip a round but not high enough to push the bolt catch to make a solid lockup.

- Worn or slightly smaller profile on bolt catch just not high enough to make a solid lockup on the bolt.

- Rifle undergassed due to misaligned or too small a barrel gas port. I doubt this is the case as you might have experienced the odd short stroking.

- Rifle over gassed and buffer weight too light, not giving enough time for the magazine's spring to push up the empty follower in time to engage the bolt catch and arrest the bolt. Got an H2 or H3 buffer kicking around to test?
Click to expand...
 
So just an update, and thank you to everyone who offered up their suggestions to help.

MD Charlton had my rifle for two weeks, and they've been a bit slow getting back to me. It's taken a few emails to get a response back. Today I was told "The Armorer has adjusted your gas port and test fired your rifle. It will be returned to you this week."

So, the take away from this, (If the issue is truly fixed) was that the problem was definitely gas related. They didn't indicate exactly what the problem was, so it still could have been a number of things with the gas port.

Hopefully all is fixed and there won't be any further issues! Fingers crossed!
 
They didn't specify, it was hard enough to get that response from them. Maybe the armorer included paperwork detailing the fix?

All the parts on the gun were stock.
 
I started having issues after replacing the gas block and rail with the bolt not locking back on an empty mag, even sometimes failing to cycle on my DD. After pulling the rail, gas block and checking everything I noticed the 3 gas rings on the bolt were all aligned with the gaps pretty much directly in line. I oiled every thing up, spaced the gas rings evenly and put it all back together - problem solved.
 
Glocked said:
I started having issues after replacing the gas block and rail with the bolt not locking back on an empty mag, even sometimes failing to cycle on my DD. After pulling the rail, gas block and checking everything I noticed the 3 gas rings on the bolt were all aligned with the gaps pretty much directly in line. I oiled every thing up, spaced the gas rings evenly and put it all back together - problem solved.
Click to expand...

The problem was more likely a mis-installed gas block since you only started having issues when you replaced the gas block.
The gas ring gap thing is a myth that just won't die.
 
Last edited:
*MALICE* said:
The problem was more likely a mis-installed gas block since you only started having issues when you replaced the gas block.
The gas ring gap thing is a myth that just won't die.
Click to expand...

Agreed, the gas rings are often blamed for things they shouldn't be. I remember seeing a bolt so loose in a bolt carrier that if you held the BCG upright, the bolt would draw in or out depending because of it's weight while you were holding it.... And the weapons fired and locked open like a champ.
 
mustang979 said:
Agreed, the gas rings are often blamed for things they shouldn't be. I remember seeing a bolt so loose in a bolt carrier that if you held the BCG upright, the bolt would draw in or out depending because of it's weight while you were holding it.... And the weapons fired and locked open like a champ.
Click to expand...

Hmm interesting. When I pulled the gas block off I noticed the carbon circle was perfectly centred around the gas port. I wiped the barrel down clean when I installed it the first time so after the ~100 rounds it left a nice little carbon footprint around the gas port. This was on a DDM4V5 that had around 2000 problem free rounds before I replaced the rail and gas block. So far after another 300 rounds since it has locked back every time. Bizarre but it's working now 100% so I'm happy.
 
You must log in or register to reply here.
Back
Top Bottom