Ive got a Colt M-16 fully auto lower issued through my dept.  It used to have a 14.5" barreled upper until i replaced it with my personally built 11.5" upper.  The previous upper never malfunctioned...ever.  When i swapped the uppers out i noticed a light primer striker happening occasionally while shooting bursts.  Usually on round 5 or 6ish the hammer would fall and the round would not fire.  When i ejected the seated round it would have a very light primer strike but would fire if i rechambered it.  No failures of any kind during semi fire (even very fast semi fire) so i'm wondering what the difference is in the two uppers that causes this malfunction.  

The malfunction didn't happen every burst but it was about once or twice a magazine (of 30.)  My boss thinks its a buffer weight problem which sounds reasonable but i wanted to get a couple more opinions and suggestions on buffer weight (if that is indeed the problem) before having him order one.  When the 14.5" upper is installed i can run 30 round mags with one burst until i get tired of shooting (or i get punitive pushups for wasting ammo...which is worth every one.)  

Any ideas?

Your boss is right. The buffer needs to be a bit heavier to prevent "bounce".
If you have a H buffer, try a H2, then if it doesnt run an H3.
HTH

The current buffer is unmarked, ive got an H buffer and i will work up the line to find one that cycles properly.  Why doesn't it "bounce" with the longer barrel?

The shorter barrel is more violent in action, spraying a bit more gas into the system. It pounds the rifle

It sounds like you have the standard carbine buffer.  The H buffer may be heavy enough to
solve your problem.

try a 9mm buffer.

don't try a 9mm buffer on a duty gun.  You need the sliding pieces inside the buffer to insure full functionality.  Go to heavier .223 buffers until the problem is solved.

Your 11.5 upper probably has an over sized gas port as a band aid to keep you from complaining to the barrel manuf. that it doesn't work.  This increases the ROF and the likelyhood of bolt bounce.

You didn't ask but I will offer this anyway as a concerned citizen and someone you might potentially be trying to save someday - since it is apparently a duty gun you should put the other upper back on and quit fucking around before you get someone (besides the bad guy) killed.  If however this is just for fun then more power to you.

thanks for the concern but please rest easy knowing that swapping an upper out isn't going to inadvertantly make the gun shoot by itself or shoot the wrong person or blow up in my hand. The upper I replaced is set for retirement soon and was a replacement of the upper before it. Thus the progression continues.

On a side note can I go too heavy with a buffer weight?  I've got range week next week to shoot and I plan on trying all these weights out. Should I look for the lightest weight that functions or the heviest one that functions.

 

I would first try an H, then an H2.  The 9mm buffer is nice (and great with semiauto-only rifles), but I have found it to be problematic with my Colt M16 when using 10.5 inch LMT and 11.5 inch Colt 933 uppers.  It runs less than 100% and this was with brand new milspec recoil springs.  It was most problematic when running the 10.5 inch with a suppressor.  If my memory serves me correctly, I believe I was getting failures to feed.  It seemed there was not enough energy to fully strip rounds from the magazine.  Both uppers have M4 feedramps, near new Colt M16 bolt groups with Crane o-rings and proper M4 extractor buffers, and I was using Colt stamped aluminum thirty round mags with Magpul followers which had previously been 100% reliable prior to introducing the 9mm buffer.  I was using a factory Colt M16 lower with a standard Colt factory fullauto fire control group.  The only new variable in the system was the 9mm steel buffer.  Once I started getting malfunctions, I immediately swapped out for an H2 buffer and subseuently had 100% reliable function.  

Granted, this is a sample of one and not anywhere close to scientific.  I now use an H2 on both M16s; one wears a Colt SOCOM M4 heavy barrel upper, the other an LMT 10.5 upper.  They run flawlessly.

What are the specs of your personally built 11.5 inch upper?  Whose barrel did you use?  

I would start with an H or H2, shoot 500-1000 rounds and see how it works for you.

It would be nice to say that my agency is willing to buy a new gun every time a barrel needs replacing or that anytime a gun malfunctions we can send it to the smith until its been completely overhauled.  The reality is our local government is in a budget crunch (probably like most) and we just can't afford to spend that kind of money.  On top of all that we have about 6 CMMG rifles that are out of service because of chamber over-pressure problems (blowing primers and cases every 6th shot or so) so all of our spares are issued out.  My current upper (a bushmaster barrel) was starting to show a major decrease in accuracy and its been slotted for replacement.  I have a very nice personal upper that has been put together recently with quality parts (BCM barrel) and shoots fantastic, much better than my previous one, so i swapped em out until i get a replacement.  No big deal.  

My current gun will probably shoot in the neighborhood of 2k rounds this week and if it shoots well then my boss and i will consider it fit for duty.  If it stops working we take it off the line till it can be fixed.  Again, no big deal.  This is the way it works for most swat officers and agencies in my area.  

Ive got a BCM barrel with F FSB, a DD bolt and carrier, DD rail, all inside a LMT upper.  Again, this things shoots fantastic and seems rock solid.

Top notch components; nicely done.  Again, I would try an H or H2 buffer and if your carbine buffer spring has a high round count, I would also replace your spring (go with a standard carbine buffer spring, nothing fancy).

Just a quick range report after two full days...the H buffer ran flawlessly.  Not one malfunction of any kind.  I even did a few 30 round dumps with no bolt bounce so i think its gonna be good to go.  Thanks guys for all the suggestions.

Good to hear!