AR-15 build problem

[deerslayerdave]

So I broke down and built my 6.8 spc upper. The problem I’m having is about 1 out of every box (factory Hornady 120 SST’s) won’t quit eject. The spent case will extract, but get jammed against the barrel with the loaded round being fed into the chamber. I’ve only had this problem with the SST’s, but haven’t shot enough of other brands to know if its brand specific. Any ideas on how to fix it?

[skizzums]

I will start with the simple answer, your gas block is just slightly out of perfect alignment. Do you know how to diagnose that? If we find that's 100% not the issue, we can go into more complicated theories

[rickyrick]

Not to familiar with that caliber in an AR, but it seems under gassed to me as skizzums noted.

[bfoosh006]

How heavy is your buffer ?

You might try a lighter one.

[marine6680]

Sounds under gassed.

If you pushed the gas block all the way against the shoulder on the barrel, you went to far, and that may be the issue. For some reason, even low profile gas blocks may need the little gap where the front handguard retainer plate sits.


If not, then buffer weight is high, or the spring is too heavy. (if you went with stronger spring or heavier than an H buffer)

[skizzums]

this is kind of hard to explain in text, someday i'll make a quick vid of this. I was having a heckuva time getting my to block right on a 300BLK pistol. so I would take to the range and bring my allen keys and just keep adjusting one way or the other til it got right. still had issues with some ammos.

when I got home, I removed the block entirely. used a wooden q-tip, broke off fluffly end. sanded it down until it was just slightly less than the diameter of the gas port on the barrel. stuck it in the gas-port, and broke it of at a 90 degree angle. took it back out and cleaned up and rounded the edges. so, now I have a stick that is the same diameter as the port, but can freely move in and out of the port. it is also the exact length of the top of port to the bottom of the barrel. I place that stick(guide) into the port. attach gas-block. look down the barrel while positioning the gas block with barrel upside down. I know that when the guide falls into the gas block I am perfectly aligned. then tighten gas block and either use a cleaning rod and just break the guide out of the barrel(I did), or if your uncomfortable with that, just use a pencil and make several index marks on the barrel and block, and circle around the barrel where the block is sitting. then you can remove the block and take out the "guide" and re-install using your index marks.(I hope this makes sense, it's actually very simple, just hard to explain) fixed my problem anyway, my gas-port was slightly off on either the barrel or the block, but this lined it up without having to fidget with it at the range.

if get 100% confirmation your aligned, you can go lighter buffer, adjustable block or even open your port by just of a gazzilionth of an inch. I had once just gave the barrel port a chamfer and that did the trick, but don't go messing up your barrel til you have ruled out everything else. i'll still bet that just aligning the block correctly will fix any future issue.

[deerslayerdave]

You were all right, I put the gas block against the barrel shoulder which resulted in it being slightly misaligned. After using a piece of nail in the manner described above, to find the correct placement I think it should fix my problem.

[tirod]

I was researching a similar issue and discovered there are gas block shims on the market now to space the block correctly.

I went from handguards with the front spacer, to freefloat and partially extracted cases jamming things up as the new round is feeding. The most likely cause is gas block alignment. The BCG isn't flipping the case out as it stops just before it gets to the port, but far enough back to catch the next round.

Gas blocks still need spacers and it's recognized by having shims out there for sale.

[rickyrick]

I found side to side alignment to be very easy simply make sure the gas tube is centered down the barrel and the gas key engages the tube easily without dragging.

The fore and aft alignment can be tricky without the handguard plate on it.
Spacers seem to be the best option

[Palmetto-Pride]

I was researching a similar issue and discovered there are gas block shims on the market now to space the block correctly.

Those gas block spacers are easy to make just take a metal cutting wheel on a grinder and cut a hand guard plate(quarter it) you can then use it as a spacer/shim that you can pull away after you tightened up the gas block.