On the last round of the magazine, should the bolt lock open if there is nothing behind the butt stock (such as your shoulder)?

I noticed the bolt wasn't locking open when shooting from a sled type of rest a few weeks ago. Today, I had the chance to go and try to solve the issue. I had a lighter spring and a lighter buffer to try. Before changing anything, I discovered that when shooting the rilfe with my shoulder behind the butt stock, in a normal shooting stance, the bolt locks back just fine. Remembering I was shooting from a sled when experiencing problems, I held the rifle to the side of my shoulder (so there is nothing behind the butt stock) It will cycle rounds just fine, but when there is nothing behind the stock, the bolt will not lock back on the last round.

I didnt take any other AR's with me, and I didn't change out the spring or buffer. I was hoping someone would know what should be happening without some force behind the butt stock before I started changing things around.

Thanks guys!

Sounds like either it is a bit overgassed or in need of a heavier buffer.  the shock from the butt hitting your shoulder is releasing the bolt.

The bolt normally will release if you give the rifle a good jar.

I might have done a bad job explaining what is happening.  When I shoot the rifle normal, as in holding it against my shoulder, everything works fine and the bolt locks back on the last round.

When I shoot the rifle without something solid behind the stock, it still works fine, but the bolt does not lock open on the last round.  I'm just wondering if this is normal, or if the bolt should lock open in both situations on the last round.  

Probably normal. I haven't done this with a rifle, but it sounds basically the same thing as "limp" wristing a pistol.  


 


 

This crossed my mind as well, the only thing I am hung up on is that I swear I have shot other ARs out of the rest and they have locked open on the last round.

None of my AR's   have ever failed to lock open  when fired from my waist un supported other than from the sling and hand on the pistol grip .

Mine also locks open when fired from the hip.

Mine also, if the "limp wristing" thing were an issue then AR pistols wouldn't function properly, which they do.

The gas only gets the bolt back so far, inertia carries it the rest of the way.  Without something solid behind it the entire rifle moves back, meaning the BCGs inertia now has to carry it back even more.  If the gas didn't give it enough oomph then it won't have enough inertia to travel back when the rifle is also moving.  So long story short, you might be a tad bit under gassed.  Lighter buffer should fix it.

I'm no gunsmith but I'll just say what I did to fix my bolt that would not lock back.  I had an issue with my bolt not locking back on the last round, I inspected the magazines and  tried 6 different ones, along with checking my buffer. But in the end it was my bolt release spring that was a little to strong of a spring. After replacing it with a weaker spring the bolt has locked back every single time on every mag I have tested.


I also year that on some rifles if you use the Magpul BAD lever that the bolt wont lock back.

First off yes your rifles bolt should lock back on an empty mag shouldered or not ,so some basic info on what your running as you mentioned lighter spring and buffer you were trying so I can get a better picture of whats going on...also what type of ammunition and bullet weight were you using when this happened just so we have all the info what type of barrel ?,Legnth ? chrome lined or not ? ,Type of rifle carbine,midlegnth or full size rifle/other type or configuration ? ,standard gas block or low pro ? , pinned or set screw? , weight or type  of buffer ? if a 9mm buffer 1 piece colt type or RRA 2 piece type?

if its not giving enough gas before answering any questions above did you check the carrier key to make sure its not a hair loose in anyway that may case gas loss?

Each rifle will be a bit different, even some that are 'identical' have variances in them that would cause one to not lock open, and the next will.
Tune your rifle so it operates under the circumstances you shoot it under and call it good.

You can create a short stroking condition if you are borderline undergassed by letting the gun travel more rearward under primary recoil before the bolt unlocks, similar to limp-wristing a handgun.
This can be especially prevalent in lighter guns shot from a bench without firm support.

This, you are slightly undergassed. I have experienced this many times when tuning an adjustable gas block.

Try a lighter weight buffer.

This is exactly how you test for short stroking. Your rifle fails the test. A lighter buffer with USGI recoil spring is the solution. If it still doesnt lock back with a standard buffer and spring then you have gas leak issues.

From the FAQ

Thanks for all the help and suggestions guys.  

When I bought this rifle I put a Red LaRue Tactical Spring in it as well as a T-2 Buffer.  I hope that the combination of the two is just a little too much.  I will put the original White LaRue spring back in.  If that doesnt solve the issue, I will put the original buffer back in.  I didnt shoot the rifle before changing this out, and the only reason I changed them out is because this is the setup on my other rifles and I really love the way they shoot and feel.  

Again, thank you for all the help.  I will do some changing around and see what happens.

Thats a pretty heavy combo for all but an overgassed 16" carbine gas upper IMO.

What Brl length and gas tube (carbine/mid/rifle) are you using?

This is a factory 14.5 LaRue tOBR rifle.  I should have probably left it as is, but like I said, I really love shooting behind this red spring and a T2 Buffer.  Havent had any problems in my other rifles, but as mentioned....every gun is different.  I need to go shoot my other guns in a manner that nothing is behind the stock just to make sure they are not having this same issue, but I believe they are gtg