AR-15 shortstroking with reloads

[392heminut]

My son bought a DelTon AR-15 from an individual over on brianenos.com and the seller said the gun ran great. We have been shooting my reloads in it (55 gr. FMJ over 21.0 grs. of IMR 4198) and it seems to be shortstroking as it doesn't pick up the next round from the magazine. We tried some of the rounds in a friends AR this last weekend with the same result. Funny thing is, I've been shooting this load in my DPMS carbine with no problems at all. My son's rifle and the other one we tried were 20" barrel guns. The load is pretty close to a max load according to my data so it kind of has me scratching my head why it would shortstroke in these guns. Should I try a faster or slower powder, or maybe try a little more of the 4198? I'm not getting any signs of high pressure with this load. What do you guys think? <dontknow>

[BLKSHEEP]

When you tried it in your friends gun did he use his own mags or your sons mags?

[washguy]

Hiya !    Put one round in the mag and see if locks back...to confirm shortstroking or not?  that would be my first step :)Wash

[98Z5V]

Hiya !    Put one round in the mag and see if locks back...to confirm shortstroking or not?  that would be my first step :)Wash

Yep, what he said...  ^^^

Might not be short-stroking at all - might be traveling WAY too fast. 

Also, try some factory ammo in it before you come to a conclusion.

[392heminut]

In the friend's gun we tried it with his and our mags, same thing. My son tried some Winchester 55 gr. FMJ's and he said they functioned fine. We also tried G.I. and Magpul mags, no difference. I'll have him try the one round in the mag and see what happens.

[392heminut]

BTW, we have tried his rifle with the A2 stock and a carbine stock with the same results with the reloads. It doesn't seem to be a recoil spring or buffer problem.

[98Z5V]

My son tried some Winchester 55 gr. FMJ's and he said they functioned fine.

You just nailed it.  Your reloads (at almost max load) were causing that BCG to travel way too fast.  The BCG was over-running the next round in the mag, before the mag spring could lift it up.

I'd bet you would find a big difference in the gas port sizes in both those barrels, if the barrels are constructed the same. The port size in that Delton needs to be bigger, if your reloads are what's going to be used.

This gas port stuff is coming up alot, lately...

[98Z5V]

Gas port diameter particulars, for 5.56 guns:

Barrel Length (in) // Barrel Diameter (in) // Distance from Muzzle (in) // Min Port Size (in) // Max Port Size (in)

11.5 // 0.625 // 3.850 // 0.081 // 0.089

11.5 // 0.750 // 3.850 // 0.086 // 0.094

14.5 // 0.625 // 8.375 // 0.063 // 0.078

14.5 // 0.750 // 8.375 // 0.070 // 0.086

16 // 0.625 // 8.375 // 0.063 // 0.078

16 // 0.750 // 8.375 // 0.070 // 0.086

20 // 0.625 // 6.875 // 0.086 // 0.093

20 // 0.750 // 6.875 // 0.093 // 0.096

24 // 0.825 // N/A // 0.089 // 0.089

For 5.56mm

Colt's gas port sizes are:

* 10.5" - .093

* 11.5" - .081

* 14.5" - .063

* 16" - .063

There's alot more gas in a 22-250, so I wouldn't rely on those numbers (5.56 stuff) strictly...  <dontknow>

[392heminut]

You just nailed it.  Your reloads (at almost max load) were causing that BCG to travel way too fast.  The BCG was over-running the next round in the mag, before the mag spring could lift it up.

The port size in that Delton needs to be bigger, if your reloads are what's going to be used.

This gas port stuff is coming up alot, lately...

I'm confused here, wouldn't a larger gas port aggravate the problem?

[392heminut]

Well, you guys hit the nail on the head, bolt is moving too fast. I tried the one round in the mag 3 times and the bolt locked back every time. Put a clipful of factory stuff through it with  no problems then tried a mag with reloads. About the fourth time it failed to chamber a round the round was partway up the feedramp with the bolt stopped on top of the round. Next step is to back off on the powder charge and see what happens. Thanks for the assistance guys! <thumbsup>

[unforgiven]

<munch>

[392heminut]

Unforgiven, you need to lay off that popcorn, stuff will give you nasty gas! <laughs>

[unforgiven]

And its messing up my boyish figure :o <lmao> But when you guys go techy can't help it I'm at the edge of my seat. <laughs> Soooo much to learn,Sooooo very much to learn.

[98Z5V]

hemi, you may not be far off, for the load - if it was WAY too fast, it wouldn't even lock back on the last, or only, round in the mag.  It would be over running the bolt stop, and not lock back.

It's hard to try to figure out what's going on, just based on a description of actions - weak rounds won't lock a BCG back, and over powered rounds will do the same thing, sometimes.  There's always more to find out, and more to ask, but I think you found out the issue here. 

Did you have any popped primers or flattened primers, with your reloads though this particular gun?  I'm sure they were fine in your gun, but what did they look like through this one, that's giving you trouble?

[Dusty44]

We just went through slowing and smoothing the BCG by adding a lead weight to the carrier.  Might be the best and easiest solution.  That thread comes complete with a Tech Order instruction.  Tried but can't remember the title of the thread and can't find it.

[Dusty44]

Didn't Hoot post the lead weight instruction Tech Order? 

Use a short piece of electrical conduit for a mold,  make a 2 inch/4 ounce weight and insert it into the back end of the bolt carrier;  increases the weight of the BCG from 5 ounces to 9 ounces and the whole thing does better forever after?  Now if I could just remember names and titles!!

Or go the hard way and buy the heavier bolt carrier?

[edgecrusher]

Carrier weight system.

[392heminut]

Dusty, I read Hoot's writeup and that is something to keep in mind. The first thing I'm going to do is back off from 21 grs. of 4198 to 20 grs. and see what happens there. It'll take me less time to knock out 20 rounds on the Dillon than it does to drive to the range. Got a USPSA match in the morning in Las Cruces so it'll probably be Sunday before I get a chance to do anything with the rifle.

[Dusty44]

Wasn't sure where you were.  Mention of Las Cruces brings a flood of memory.  Holloman and Alamogordo in the middle 1960's!  Those were some of the good years!  Two of my babies were born at Holloman.  Would take my then new Colt M1911 out to the base of the mountains on the East side and had all the room in the world to shoot with a 5000 foot high backstop.  I have been told that now all that area is heavily developed,  houses and people everywhere(?).

[392heminut]

Yup, it's getting so you can't throw a rock around Alamagordo or Las Cruces without hitting someone. Sheesh, it's even getting to be that way here in little ol' Deming. We had a boom here shortly before the housing market crashed, a lot of folks from California moved to this area. Can't say as I blame them!

[392heminut]

Well, problem solved! After screwing around with the IMR-4198 until I got PO'd with the metering inconsistencies I went to the local gunshop and picked up a pound of H335. I went home and knocked out 30 rounds at 25 grs. even, loaded them in a surplus mag and went to the range. The gun ran like a top, not one hiccup! The H335 meters great too!