At-15 cycle issue

[Nordic95]

Hey guys newbie here to the forum. I just recently built a 450 bm and I’m having issues with it cycling properly. It’s a Davidson’s Defense upper with a SAA BCG. I use PSA lower for this build with PSA lower build kit. I started out with an H1 buffer at 3.4oz and it would eject brass but not strip amd load next round. So I installed an H3 at 5.4 oz and same issue. I don’t have an adjustable gas block and I’m using two different magazines but from same company. They are steel 450 BM magazines. I’m using Double Tap amd Hornady ammo for testing and break in. So far I have about 30 rounds thru it. The Douple Tap ammo actually did lock back the action a couple times as I was only loading one round at a time. I just wanted to see action was working but only 2-3 times it actually locked open. Today I tried some reloads and the same issue persists. What should I be checking next?

[Al in Mi]

welcome.

I'd oil the snot out if it and run it wet, then check gas block alignment. and if you happen to pull the block off, try and measure the port size.

I don't ever recall reading or hearing of a "standard" port size for carbine or middy length gas systems.

[plant_one]

i doubt its a lower issue - but if you've got another, or can access one thru a friend - its always worth a swap to see if the issue goes away.

it might help recording the shots to see if your BCG is cycling fully, but it sounds like it is at least some of the time.



without knowing what company's mags you're using, we wont be able to know if they're on the good or bad list. that info will be useful.



it does appear by looking at their site that they only sell 16" uppers, and the specs show carbine gas system, but they do not give their port dimension.

i'd probably suggest as noted to dissassemble the upper, check that the gas block is correctly aligned, and while you're there to get a port dimension.

you're looking for a good clean carbon ring completely around the port hole
here's some pics from the web showing what a good & bad alignment looks like based on the carbon ring

good carbon ring showing solid alignment.


if th carbon ring looks like the one on the right - or even worse where it intersects more of the hole - you've got an alighment issue to address


i would also look for any signs of leaks around the gas block. you'll see carbon coming out if there is.. its pretty obivous when you look for it




also note - most low profile gas blocks should have a small gap between the block itself and the shoulder on the journal closest to the chamber. the journal and gas port location are usually cut to accept a traditional plastic handguard with na a1/a2 style front sight post - and those have a small peice of metal there to hold the front of the handguard in place. if your gas block is touching the shoulder - its probably blocking at least SOME of the port.

quick pic again from the web showing the small gap there.


again these are just some examples of things to look at while you're in there.

HTH

[Nordic95]

Thanks for the advice so far. But I still cant get this thing to cycle properly. First thing I did was upgrade to an H3 buffer from a carbine. Went back to range and three different ammos and two brands of mags still the same problem. Last night I completely torn down the upper and cleaned and polished all mating surfaces on the bolt. I polished the feed ramp and blended the upper into the feed ramp. I removed the barrel and lapped the upper, figured since I'm tearing apart I should do everything I've been wanting to do. I also added an adjustable gas block and checked alignment to the gas port. Today I tried same three ammos and mags all while adjusting the block from 1 turn to all the way out and it still FTF and no lock open. Actually only twice since I bought this rifle has it locked only out of 70 rounds so far. I'm using double tap 300 gr and these are what locked the bolt back twice. Also using Hornady white tail 245 gr and hand loads of 225 FTX over 38-40 grains of lil gun. Only thing I can think of is gas port hole size is to small and I'm about to tear apart a measure it now.

[Nordic95]

Just checked gas port hole. I dont have numbered drill bits but .069 was a little loose and .077 would barely start to enter the hole. Im saying its a .075 so should I open it up some and to what dimension?

[Al in Mi]

My 16" factory Bushey has a .093 port, and has never had a hiccup.

[s4s4u]

Adjustable gas block does no good if you are already light on gas. Do you have a standard carbine buffer? My Bushmaster barrel also has a .093" port. If you now have an adjustable gas block it won't hurt to open the port a smidge.

[Nordic95]

I just drilled it to .091. I’ll see tomorrow what happens.

[Nordic95]

Todays update....

So last night I drilled the gas port to .091. I went to the range today with some reloads and a second lower to try out and see if the lower was the issue. I installed second lower, opened up gas block 4 turns and tried a 225 FTX with 38.0 LilGun. It didn't cycle so I opened up to 5 turns and then they started cycling and locking back on last round...HURRAHHH. so I switched lowers back to the original I built for this gun and they cycled a full 5 round mag and locked open...HURRRAAHH.. So I moved up to some 225 FTX with 40.0 LG and shot some more rounds as well as some 250 Barns over 34.0 LG. All cycled fine except I had three out of 50 that didn't extract. I opened the bolt and the brass fell out so it wasn't expanded but it did have extractor marks on it. What would cause this? Remember I have an H3 buffer in this lower, maybe too heavy? I'm 90% there to getting this thing to work properly and im excided as its a fun round to shoot and load. Thanks for the help so far guys....

[plant_one]

h3 not too heavy. promise.
i'm running h3+4oz slug (Hoots CCWS) added to the rear of the BCG. like butter.

i've also run the same upper on a lower with a 8.5oz XH-carbine buffer from slash's. that lower was intended for blowback use (9mm) but also works nice for big bore/straight wall useage too

small edit Hoot