hey guys.  i know somewhere on here this has been addressed before, but i couldnt specifically find the topic in search, so i'm asking for help.

my current setup-

20" sabre SS 223 fluted heavy barrel
Vltor Mur upper
Vltor gas block (small, fits under quadrails)
tactical innovations lower w/ colt lpk
MPed bolt from PK firearms
badger ordnance charging handle (gen 1)
a2 stock

ive had this gun previously assembled with another bolt and charging hanlde, and it worked fine.  i used those two parts in a carbine i built to sell a while back, and finally got my new stuff today in the mail.  the bolt is an MPIed unit from PKfirearms.com, and so is the badger ordnance charging handle.  two things i noticed when i was function checking it (manually loading each round out of a 20rd mag):  it feels really tight, regardless of how much lube is in there, and also that the charging handle has some very evident drag marks on it (where the anodizing had worn off from the manual loading).  i thought it would be no big deal, so i tried a couple shots.  the first shot fired, extracted, but short stroked the would not cock the hammer for bullet #2.  i dropped the mag, extracted the bullet, cocked the hammer, then reinserted the bullet and it fired and extracted properly.  im using PPU 69gr SMK match bullets by the way, and ive never tried them in this rifle before.  ok, so i reinsert the mag, charge the gun, fire and two shots run concurrent, as in slam fire, both shells extract, and the next bullet is loaded but the hammer would not cock itself on the backstroke.  i tried this 3 more times, and it does the same thing:  two shots with one trigger pull, loads the third but cant cock the hammer on the backstroke.  the only thing i have changed on this rifle since the last time it ran (and at that time had a 0% failure rate).  i had to remove the gas block to fit my KAC m5 ras rail system, and that is the only thing i have dicked with since the last time it ran.  it is possible i dont have it exactly right, and i think that could be my problem.

anybody else have an idea other than that?  i just want to make it to the range this weekend, i have a buddy with a bolt action i need to embarass, and i'd hate to do it one shot at a time.

OK, uh, sounds like a couple of issues here...

First do a visual inspection...is anything obviously amiss? Do you have other BCGs to try? Is there any debris visible on the firing pin or inside the bolt?

Is the hammer cocked without issue when you pull the charging handle back by hand?

Anything worn or damaged in the trigger/hammer/disconnector area?

I'd like to add that when you perform the second step in 87GN's questionnaire, that you (with a cleared rifle) depress the trigger when you do this... It doesn't seem your sear is causing the issue, if anything it would be the disconnect. It won't engage without the trigger depressed during charging.
It isn't the charging handle... That wouldn't likely cause a failure like this.
Aside from that, like he says, it would be great if you could try another BCG... Since that is the part that has changed which might be causing the failure... Did you change just the bolt or the whole BCG?
You never experienced doubles with the rifle in its previous configuration, did you?
 

thanks for the input guys.  yes it was the entire BCG that i changed out, and its one of the $150 deals for PKfirearms they are currently selling as MPI ar15 bolt.  i dont know what brand.

no, i never experience anything but 100% reliability prior to this.  i dont have another bcg i can try at the moment, but i might could borrow one for trial purposes.  i know all the lower parts are in good working order because i swapped my lower on to a buddy's rifle while i was doing the testing, and it function fine.  the trigger and all the lower parts are new with low round count, and all seem to function like they are supposed to.  everything looked good when i did a visual inspection and nothing is bent or looks out of place, and there isnt any excessive wear on anything.

the hammer does cock when you pull the charging handle back manually, and the rifle fires and functions like its supposed to when one round is loaded. . . including the bolt locking back on the mag catch after the last round (which happens to be the only round).

do you guys think it could have anything to do with the gas block being moved like i stated above?  like maybe i have it off just enough that its getting lower pressure?  also, could it be that the PPU ammo has light primers?  i noticed today after checking out some of my bullets that every third round (the one that didnt fire during my initial function tests) all had dimples on the primers.  

i may call PKfirearms and ask what brand BCG it is they sold me, since there are no marks on it except MPI.  as good as theyve been in the past i imagine they'd swap it for me it is in fact the problem.  who knows.

i spent some time today when i got off work with it.  i cleaned it again and cycled it manually with some light lapping compound.  it loosened it up after about 100 cycles, and then i cleaned it again and checked the inside of the rifle for flaws or light spots, which there were none.  it appears the problem is not the upper, which i already knew, but i wanted to cover all my bases.  i check the chamber again as well, and it is just as it should be.  im probably going to try and get a couple boxes of other brands of ammo and try them as well just to be sure its not the PPU.  

keep the ideas coming guys, im going to work on it some more this weekend.

The dimples are completely normal...



It might be the gas block... I didn't see that earlier. It seems like a very likely culprit.



Trying different ammo certainly won't hurt... I'd recommend something pretty hot like M193 or M855.

thats what i was leaning towards.  i wish i could find some MK262 ammo somewhere, thats basically what my barrel is made for.

im going to check the gas block out tomorrow when i get a chance.  i'll see if that helps.

"my current setup-
20" sabre SS 223 fluted heavy barrel"


You have a 223 Chamber, Do not use 5.56 Ammo.

thanks, but i already knew that.  there are a few companies that make mk262 clone ammo that is 223.  

well i took it apart and found that the gas tube was bent ever so slightly (must have done it when i was removing and reassembling before) and all it took was a little persuasion and it went back straight.  that cleared up my doubling issue.

now it just short strokes.  it will fire, eject the spent casing, and load the next round, but will not cock the hammer.  ive been through it a few times and i still havent had any different ammo to try, so that may be my problem, im not sure.

oh, and i did check the gas block;  it was off a butthair from where my set screws were previously, but my extremely minor adjustment i made had no affect on the short stroking.

i need hot ammo!

ok, now im totally confused.  i recleaned and relubed again (this makes 3 times) and checked fit, and everything seems to be doing what its supposed to when cycled manually.  i loaded a mag of 5 rounds, charged the gun, pulled the trigger, and my gun proceeded to dump all 5 faster than i knew what had happened.  i tried it again over the course of 30 rounds, and 3 times it ran all 5 out.  the rest of the time it was single shot, spent shell ejected, loaded next round, hammer would not cock.  now i noticed that the gun only has to stroke back about 1 inch (or around half the length of a spent case) to recock the hammer when you are manually cycling the gun.  why the hell is my hammer not resetting?  could it be the bottom edge of the BCG is not tall enough to make the hammer click into the disconnector?  

this is starting to piss me off.  whether my ammo is crap or not i shouldnt have this mess if it cycles the bolt at least half way back.  

somebody chime in, im running out of ideas, and i dont want to have to send this BCG back and wait another 6 months to find one.

on a side note, how are you using a low profile gas block with a KAC M5 RAS that is designed to mount like standard HGs?

Dude, if you tell the company that you bought it from that you have a malfunctioning rifle after installing their BCG, a rifle that fires more than one round per trigger pull, they will do everything humanly possible to fix that situation in a very rapid manner, perhaps not as fast as your rifle shoots but almost as fast.

how am i using a m5 ras with a low pro gas block?  i installed the triangle handguard retainer behind it and tightened it down.  

ok, i fixed it.  how about a broken disconnector spring?  yes, my brand new lpk had a disconnector spring that was either weak or improperly heat treated, and it failed.  end of story.  rifle functions beautifully and shoots sub MOA now.  im in love again.

time to build another.