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Built my first AR - shortstroking?

M

madd0c

Sergeant
Full Member
Minuteman
Mar 6, 2010
453
0
54
South Carolina
I finished assembling my first AR from a stripped lower last week. Took it out today and was very happy with the results.

It fires and feeds reliably, no failures, but it will NOT lock open on an empty mag. Is that an indication that the gas block is not properly aligned?

If that is the case, how do you know the gas block hole is aligned to the barrel, it seems that it is a blind seating, and I just leveled the gas block to the reciever using bubble levels.

Is there a better way to do it?

Thanks,
madd0c
 
Re: Built my first AR - shortstroking?

Not locking the bolt is often indicative of bolt bounce. This is caused by high bolt cycling speeds. You most likely need to install an extra-power recoil spring or a heavier buffer to slow the bolt down.

This is all assuming that your magazines are in good working condition.
 
Re: Built my first AR - shortstroking?

Are you using PMAGs? If so, pull the upper off and insert an empty mag. Make sure the follower is making contact with the bolt catch.
 
Re: Built my first AR - shortstroking?

yep, using pmags.
the pmag I was using worked fine in another AR.
I will check and see about the bolt catch.
If that is not it, I might throw in another recoil buffer or spring, the ones in it are standard DPMS carbine buffer/spring.

Thanks for the info.
madd0c
 
Re: Built my first AR - shortstroking?

The bolt catch seems to be in contact with the follower.
I ordered one of the extra power spring kits, we'll see how that does. Definitely a cheaper option than the buffer if it works.

Quick question to you all, is the way I installed the gas block the right or only way to do it? there isn't a tool or some trick to check hole alignment other than leveling it?

Thanks,
madd0c
 
Re: Built my first AR - shortstroking?

hmmm...I usually think of bolt bounce on the other end of the cycle, and usually under FA conditions.

Interesting diagnosis without more info: ammo being used, barrel length, gas system, whose barrel, what kind of gasblock, etc.
 
Re: Built my first AR - shortstroking?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sobrbiker883</div><div class="ubbcode-body">hmmm...I usually think of bolt bounce on the other end of the cycle, and usually under FA conditions.

Interesting diagnosis without more info: ammo being used, barrel length, gas system, whose barrel, what kind of gasblock, etc.



</div></div>

DITTO..............what ammo, check gas rings and then check alignment of gas block. Use a known flat surface, I have a block used for kitchen countertops made of granite.
When you check everything, put 1 round in a mag and fire it, see if it locks open.
One other thing is to ask is what buttstock and buffer/spring combination you are using.
 
Re: Built my first AR - shortstroking?

This is just my opinion, but I never run anything but a milspec buffer spring. Going with a heavier buffer spring will slow down the bolt coming back, but will force the bolt forward with greater speed. If you decide that you need to slow the bolt down, go with a heavier buffer.

First, have you taken note of your ejection pattern? Is it throwing the brass forward at the 1:00 to 2:00 position? The 3:00 position or back at the 4:00 to 5:00 position. This can be an indicator of your gassing situation. Usually, the farther forward your brass is being thrown, the more gas is being dumped through the system. How far out is the brass being thrown? We just need to get a read on your gassing situation.

When you built the rifle, did you use a free float handguard? If so, is the gas block sitting right up against the shoulder of the barrel? There should be a 1/32" gap between the gas block and the shoulder of the barrel to allow for the handguard ring (that isn't there if you used a free float handguard).
 
Re: Built my first AR - shortstroking?

What's your gas block? Standard front sight or adjustable.

Rich
 
Re: Built my first AR - shortstroking?

They make 2 kinds of gas blocks.1 has to have a gap the other shoulders up.If you have the paperwork check it out or you can use a caliper to measure the distances.
 
Re: Built my first AR - shortstroking?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 1Sg (ret)</div><div class="ubbcode-body">They make 2 kinds of gas blocks.1 has to have a gap the other shoulders up.If you have the paperwork check it out or you can use a caliper to measure the distances. </div></div>

Which gas blocks are they that allow shouldering up? I've never seen one that makes the distinction.
 
Re: Built my first AR - shortstroking?

Ok, wow, bunch of posts since I ordered a spring kit.

It is a dpms upper with a NATO standard M4 profile 16" barrel.
I took off the A4 front sight/gasblock and put on a DPMS single pic rail standard barrel 0.750" gas block - non adjustable -after mic'ng the barrel.
Used a YHM carbine length free float hanguard and there is a good 1/2" between the rear of the block and the ff handguard.

Ammo, both Black Hills blue box 55gr FMJ and handloads:
Berger 70gr VLD with Varget 25.3 gr seat 2.260"
SMK 69gr BTHP Varget 25.3gr seat 2.260"

had the same not locking open problems.

brass ejection: 1 o'clock and way the F$%^ out. Like six or seven feet.

I will try the spring since I already have it on the way.

Thank you for all the input,
madd0c
 
Re: Built my first AR - shortstroking?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TonyAngel</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 1Sg (ret)</div><div class="ubbcode-body">They make 2 kinds of gas blocks.1 has to have a gap the other shoulders up.If you have the paperwork check it out or you can use a caliper to measure the distances. </div></div>

Which gas blocks are they that allow shouldering up? I've never seen one that makes the distinction.</div></div> I have 2 that are yankee hill i think.I worked at a custom AR shop for while and we did a bunch of rail conversions.Veltor Makes 1 also but i cant remember if it needs a gap.They my have changed this over the years.I was just giving the op something to check.Thanks
 
Re: Built my first AR - shortstroking?

Ok, if it's ejection pattern is throwing the brass that far forward and six to seven feet out, I don't think it's a gas problem. Really, all I can do is venture a guess, but I'd say that you have a LOT of bolt velocity going on there to punch the brass out at a 1:00 position. In fact, I'd say that you have too much bolt velocity.

Without being able to lay my eyes on the rifle, I have to ask, what stock are you running? Rifle length buffer tube or carbine length buffer tube. Are you SURE that you're not running a carbine buffer/spring in a rifle length tube.

What kind of lower are you running? Have you confirmed that everything in the lower is right by running it with another upper or is the lower a part of a fresh build? Are you sure that the bolt stop is operating smoothly? Is it binding?

Sorry for all of the questions, but it's hard to diagnose on the net like this.

Oh, can you manually lock the bolt back? I know that lots of guys like to go with heavy buffers and springs. I run heavy buffers in all of my carbines, but that is just to fine tune or tailor the way the rifle shoots. You should NOT have to add a heavy buffer or spring to fix a problem. If adding one or the other makes a symptom go away, all you've done is mask a symptom. You haven't fixed anything.
 
Re: Built my first AR - shortstroking?

What lower, and does it lock back with a different upper?
The upper was unshot before you took it apart, or was it running fine before the handguard swap?

Could be as simple as a bolt catch spring, you never know. Everyone always jumps to big problems, but I've seen little things pop up after hundreds of dollars worth of fixes have been applied.
 
Re: Built my first AR - shortstroking?

Using the same buffer, spring and buffer tube that functioned fine before the switch from the A4 to the low-pro gas block. All are carbine spec. All I did to the buffer end was add a magpul MOE stock and an endplate QD mount from Noveske.
The bolt spring/catch worked fine until the swap and does not appear to be binding. It moves smoothly, and yes I can manually lock open the bolt with the catch. The lower is a High Standard marked Continental Machine Tool lower with a DPMS part kit in it.

Any way that the new gas block could actually be over gassing compared to the A4 sight block? Just thinking about where and how far it threw the brass has me going with the excess bcg speed idea.
Prior to the swap it laid the brass out at 3-4 o'clock at maybe 4 feet tops.

Thanks again for all the help,
madd0c



 
Re: Built my first AR - shortstroking?

I vote gas rings, send me a pm and I'll mail you a mcfarland 1 piece ring I have laying around so you can try that.
 
Re: Built my first AR - shortstroking?

I don't see how it could be a gas ring problem with the rifle flinging brass like that. Still, I really hate to start rattling off at the mouth without being able to put my hands on the rifle.

OK, I had to go back and look at the original post. It's the lower that is a fresh build and you put a new gas block on the upper and you CAN manually lock the bolt back. This low pro block that you are using, is it a set screw type or a clamp type?

Inspite of my question about the gas block, my knee jerk is to think that something is up with the bolt stop, although the question of the increased bolt velocity still lingers. If you have the old gas block, compare the size of the port in the old one and the new one and see if there's a difference.

You problem may very well be some form of bolt bounce. Going from a 3:00 ejection pattern to a 1:00 ejection pattern just because you swapped blocks kind of has me scratching my head. I suppose it's entirely possible that there's something about the lo pro block has increased the gas pressure being sent to the bolt.
 
Re: Built my first AR - shortstroking?

Just a thought, but have you checked your gas key? Everything's nice and tight, screws staked?
 
Re: Built my first AR - shortstroking?

SobrBiker is right. It always seems to be the tiny stuff that gets you. Then again Ive had some strange malfunctions on my AR. =/

Had the catch of the ping pong paddle break once. I didnt even realize thats what the problem was until I completely disassembled the rifle twice with no resolve. (o_O)
 
Re: Built my first AR - shortstroking?

Ok, I put in the new buffer spring and am going to test it this weekend. I will update.
The gas rings appear fine. the gas key is tight and staked properly.

madd0c
 
Re: Built my first AR - shortstroking?

madd0c,

Do this step by step.

1. Is the bolt locking back manually? If not you do not have a gas problem. See if the bolt lock spring is too strong or if the bolt is able to come back all the way. You may have the wrong length buffer or spring in there. It gives extra power but won't compress enough to let the bolt get far enough back to lock.

2. if the bolt locks back manually then you have a gas problem. The gas problem may be:
a. the gas port not aligned with the gas block. You check that by leveling the rifle, then the block. Now look at the block from behind and think of a clock. Each number on a clock is 30 deg. So, between 1100-1200 and 1200-0100 there is thirty degrees, bisect that in thirds mentally. Each "minute" on a clock is 6 deg. That is the rotational distance you should be within. Within that you will be in full gas flow, between 5-10 degrees you will get a slight cut-off of the gas. Anything more than 10 degrees you are getting way too much gas cut-off. The reason it can be off just a bit is the hole in the gas block is bigger than the one in the barrel. It can slide a little left or right and still be completely within the gas block hole. <span style="color: #3333FF">Edit: Also make sure the block is back against the shoulder</span>
b. If the gas block is where it is supposed to be, then you may not have enough pressure going down the barrel. What rounds are you using? Remember it takes slightly more pressure in a 5.56 chamber to function than a .223. What do the cases look like? High pressure signs? or nothing really? Do the empty cases chamber right back in? If they don't even come close to chambering you may be using too slow of powder, which is causing high pressure down the barrel which in turn causes the "bolt bounce". Your fired cases should just barely chamber and they shouldn't have high pressure signs. This is one of the trickiest parts of reloading for an AR. Also, along with this make sure you aren't using 5.56 loads in your .223 Chamber. Hot, hot, hot...
 
Re: Built my first AR - shortstroking?

Well,
Just in case someone else gets this same problem, I suppose it was bolt bounce, because the new spring fixed it completely.
No problems with locking back after last round now.

Thanks for all the tips,
madd0c
 
Re: Built my first AR - shortstroking?

Good thing I scrolled down, was about to go through my experience with Bolt bounce... and how I turned a few hairs gray trying to get a reliable lock to the rear on last round...

Always run a good quality spring, and the strongest your ammo will allow with reliability. I want that BOLT to slam forward with as much force as it can every time.
 
Re: Built my first AR - shortstroking?

An update:

I can now report back what I think was the root cause of the malfunctions I was seeing, because after replacing the spring on the malfunctioning gun I originally posted about, I began having failure to lock open issues with 2 other ARs.

The rifle I posted this thread about soon started having failure to lock open issues again. Then two seperate ARs also started having issues. This was about the time it got colder.
This past weekend, I went shooting when the temps were in the upper teens here and it was icy outside.

The gun I originally posted about would not even FEED from a pmag, GI mag or an AR Stoner mag.

Luckily, the person I was with knew what was going on. He asked "What the heck are you using to lube the bolts with?". I was using Tetra gun grease. The tube says -100 to 750 degrees Fahrenheit, and I have had great results with it during the summer, I just used a thin coat on the bolt.

To keep this short, I used carb cleaner and cleaned all the gummed tetra grease off the bolts after complete dissasembly, lubed them up with CLP and the problems in all the guns went completely away.

I think adding the stronger spring let the rifle I originally posted about push through the gummed up tetra when it was cold, but when it got really cold (for here), it started showing the signs again.

Live and learn, and I know there are gonna be tons of people saying no way this is the cause, but I do know that in THIS case it was, CLP and carb cleaner has all of them running smooth.

I have had great results with tetra when it was 105 degrees on the range, and shot one of the rifles in a local comp running it with no issues when it was hot.

Hope this helps someone,
madd0c
 
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