Pressure Problem

[outdoorjake]

I have just finished, well am trying to finish a new AR build using a BHW 18" rifle lenght gas system 5.56 barrel. Everything went together smoothly then I get it together and I am having what I thought was a short stroking problem. re-adjusted my gas block, changed out gas tubes, put in 2 other BCGs from other rifles ( both a light weight and an M16) changed gas blocks but still having problems. Then I thought about about the rifling of your barrels and wondered if they have a tendency to cause more back pressure and that instead of lack of pressure if I am just getting too much on my impingement system and causing my bolt to run too fast.

I have been building custom ARs going on 10 years now and have never had this much problems getting a new one to run, any ideas or suggestions

[everrest]

I found this article on trouble shooting.

I would recommend lubing the BCG and hand cycling the bolt many times. Also try different magazines, buffers, and extractors.
If you can provide more details on the build I'm sure other can provider more advice than me.
BHW barrels tend to produce faster fps so I would think that back pressure would be less.

[outdoorjake]

Build Details

Barrel is a BHW 5.56 18" with rifle length gas system
gas block at the moment is a Sadlak low profile
Muzzle Break is a AAC non mount muzzle brake
upper is a RRA match grade
BCG is a PWS impingement
buffer and spring is a standard m4
stock is a Ace Socom
gas tube is a DPMS
trigger is a Timney AR-15 3lb skeleton
Mags tried PRI 20rd, Colt 20rd, Double Star 30rd with magpul followers and SS springs, MagPul P mag 30 rd, and 100rd beta mag.

the gun has been lubed and stays lubed. have cycled and shot around 150 rds through the barrel so far. have fired with one bullet in the mag to see if it would hold open on all the mags but fials to do so, have used everything from m193's, m855's hornady varmint loads, hornady match loads, remmy soft points, 223 fmj, 68 and 75 gr reloads.. nothing will cycle once in a blue moon it will cycle 2 but thats it and most of the time if it will chamber another round it is not reseting the hammer

[everrest]

Other thinks to check other than the items in the trouble shooting guide.
Make sure that the gas block and barrel exhaust ports are aligned along the barrel length.
Check that the gas port hole on the barrel is clear.
Check gas port dia. With a drill shank.
Try a different lower.
One person in another forum with a different barrel, recommeded cleaning/polishing the chamber because the extra force needed to extract a round prevented the bolt from cycling. ( I would contact BHW before doing this).

Edit
Check that the bolt catch is not dragging.

[outdoorjake]

hey everrest
I meant to bring this up on the last reply but forgot due to it got busy at work and my mind time traveled or something but where you said that they produce higher velocities and that would produce lower gas pressure, well in all my experience working with firearms and polygonal rifling on barrels in pistols they produce higher felicitous due to the bullet is tighter in the barrel and there is the lack of room in the lands for gas to escape past the bullet thus producing less velocity and lower pressures because normal rifling ask kind of like a safety valve in a way of lowering the pressure behind the bullet, that was kind of a bad example but i can't think of another way to word it.

So in that theory is where i am coming up with i think my rifle is over pressured due to when the bullet it traveling down the barrel no gas is escaping past the bullet which makes it go faster as you said, but in turn when the bullet passes by the gas hole in the barrel that is the only place for ALL the gas behind the bullet to go and in turn sending way to much of the burnt gases down the gas tube and running the bolt way to fast and not letting the pressure in the chamber to subside so that it is hard for the brass to be removed. I could be wrong but from all that i have dealt with while building and shooting black rifles that is what i am thinking.

If anyone from BHW could look at this and let me know if they have had a similar problem and how to fix it It would be appreciated

[everrest]

You have more experience than I do. I found several articles searching for "AR-15 pressure curve" and also port size. Maybe they will help. Following is one:
AR gas operation
I would recommend changing the title to cycling or FTF issue.

[GLSHOOTER]

I chickened out on the gas block issue on the 20 LBC I'm testing. I had never worked with poly rifling and in combo with a new round I wasn't sure of port pressure issues. I put a KIES adjustable gas block, very reasonable $$'s, on it and started shooting. It tuned in quickly to the ammo I am running and I think it would be an ideal way to address this issue.

The KIES is not Pitcanny railed so that might be an issue. Stepping up for more $$'s the JP unit is available with rails.

Greg

[Deleted]

one must start with the port dimension and work towards the bolt one thing at the time to identify issues with improperly gassed systems.
The adjustable block is a great way to tune your system but it is too bad most do not come with a set of preset positions that allow a quick change. I think Noveske are the ones that can do this but I have not used them.
I love the feature in the AA pistons that you have 3 modes. Open, Suppressed and Cancel.
The M4 has no issues shooting M193 and M855 ammo (LC) in suppressed mode but then for example PCM bronze needs full open to cycle as this is a lot milder.
I think the suppressed would even need 2 settings itself to adjust dwell time.
A properly gassed AR will be spitting the brass at 4-5pm in a 35-45 degree angle upwards.
Thanks.