New AR fail to fire, Doubled once

[94user]

I am perplexed to say the least. Yesterday was the second timeout with my AR. Brand new DPMS AP4 5.56 and a Stag 5H 6.8 . The 6.8 was fired 15 times to sight in a scope at a 25 yard indoor range with no problems. The 5.56 was unfired until yesterday. I had two failures to fire with the 5.56 using Federal XM193F . Full primer indentation and neither round fired when i tried them again. Two rounds out of twenty. I then ran through forty rounds of American Eagle XM193C with no problems. Is this not basically the same round? I dismissed this as bad ammo .

Now on to the 6.8 . I had four fail to fire, again with full primer indentation and three of the four did fire on a second try. This was SSA 110 Sierra pro hunter. Four out of twenty five and somewhere in the middle of this the rifle doubled . I was loading Three in the mag and the first shot fired then on the next pull of the trigger BOOM BOOM !

This has me confused as hell. This rifle is brand new, nothing done to the trigger except a generous lube with Tetra White grease. Second time shooting it . sixty round total on the 5.56 DPMS upper, Forty total on the stag 6.8

WTF? Got to be a problem in the FCG, or so I assume.

[Alleycat72]

Is the hammer resetting every time? You may have disconnector problems. If the hammer is following the bolt it would explain both symptoms.

[94user]

I found that I put the hammer spring in backwards after I took it out to grease the front of the trigger so i was getting light hammer strikes .

Don't see how that could have caused the 6.8 to double that one time.

[LngRngShtr]

The Disconnector spring is tapered,.. if it is put in backwards the gun will double,.. as far as the failures to fire, sounds like something is dragging and slowing the hammer down or the gun is not fully into battery and cushioning the hammer fall... I have heard all sorts of break in procedures but "run em wet" was the recurring theme, Uppers need oil too.... oil it down,bolt lugs,carrier pin etc. with Break Free or similar light machine oil ( I have heard of some folks breaking in with Mobil 1 oil of a light viscosity ) and see if that helps the next few mags.... no need to be dripping oil all over the bench but you will probably get some back through the charging handle raceway so wear glasses and bring a rag...

John

Edited October 13, 2010 by LngRngShtr

[Magiccarpetrides]

Too bad firing two shots at once on a non registered gun is illegal...otherwise thats pretty cool ;-D Sorry to hear about your gun problems...luckily it sounds as if they are user caused and quickly fixable.

[94user]

I beleive the double tap may have been a bump fire. I was holding it really tight on a sandbag rest. It also only happened with the higher recoiling 6.8 round.The dis connector spring was never removed and it catches the hammer every time on a function check. Now that the hammer spring is in correctly I doubt I'll have anymore FTF 's.