The upper is your issue. Going to heavier and heavier buffers may compensate, but heavier buffers also introduce more stress on the host firearm, and are not the best solution for an M16 used in a life-or-death situation. Heavier buffers can and have disabled M16s.

You have bolt bounce, which is caused by too much gas cycling the system and bouncing the bolt away from the chamber at a critical time in the full-auto cycle. OTOH, over-gassing rarely causes issues in semi-auto firearms, though under gassing will -- so many manufacturers whose primary market is for semi host guns will lean toward more gas. And AFAIK, Del-Ton does not fill military contracts, so they don't have to be concerned with over-gassing.

A heavier bolt makes up for too much gas, but you are increasing the cycling mass and weight, and that takes a toll in wear. You are asking the host gun to handle stresses it was never intended to.

One solution would be to get an adjustable gas block and dial it way down, to where it runs reliably with an H or H2 buffer. That would also give you more flexibility should you ever decide to mount a suppressor (which also affects gas levels).

However, my recommendation for any M16 which is going to be used in crisis situations, like SWAT operations, is to go to a full milspec upper made by a milspec contractor.

FWIW, I have an M16A1 made by Colt on March 7, 1978, so it's very close to yours. I've had it for 20 years and presently run about 15 different uppers on it, from 7" to 20" LMG. Except for my Shrike and LRM M169 uppers, all are factory Colt; almost all were bought new; and all were 100% reliable out-of-the-box in full-auto. I do swap around buffers ... but it is solely to modify the rate-of-fire.

I don't like unreliable guns. I get really pissed off when they break, or refuse to run, so I stay away from components and configurations that make them do so. And I shoot for fun, not because it's my profession.

Good luck. And stay safe.

I'll second everything that david05111 says.

FWIW, I have two standard pieces of advice for new M16 owners, that also apply to you IMHO.

--First, start out with brand new hammer/trigger/disconnector springs. Colt springs -- not Del-ton, DPMS, whatever. You are putting them in a Colt M16, and Colt standards are the milspec.

Normally, this is because you don't know how many rounds have gone down the tube. But in any case it's a good idea because the hammer/trigger/discon springs in yours are 36-37 years old, and even sitting in a box, springs can either weaken or take a set.

Springs are like the oil in a car or truck: They cushion the fire-contol components during cycling, to eliminate or at least reduce wear. I change mine every 5-10k rounds, just like the oil in my truck. A set of 3 new Colt springs from Brownell's is about $10.

--Second, when you are having issues after a change in configuration, return to exactly the way it came from Colt. Put everything back on that was on there when you opened the box, after detail-cleaning to make sure any 37-year-old oil or grease hasn't caked up, then lube it to mil spec. Test-fire. If it doesn't run reliably, consistently, then talk to Colt or an armorer.

If it runs in the original configuration -- and I know of no earthly reason an NIB Colt M16A1 from the mid-seventies would not -- then you start changing one component at a time. Test-fire. When it stops running reliably, you know that last component you installed is the culprit.

Finally, I stick to Colt parts and components not because I worship The Pony, but because Gene Stoner's AR15 design is tolerance-specific, and Colt pays close attention to tolerances for all their parts, to keep them from the Dreaded Tolerance Stacking, where .001 here and .001 there eventually adds up to something that does not run reliably.

Different manufacturers use different tolerances, but as long as they all add up correctly, you're OK. Knight and Noveske build incredible guns, but mix their components and you might very well have issues.

Stoner designed the AR15 as a full-auto rifle. Once a semi-auto conversion hit the market, and started being cloned by other manufacturers, the need for the tolerances demanded by full-auto cycling went out the window. And if the other manufacturers want to sell guns for less than Colt prices, they also need to cut production costs. If they are only building semis, they can use parts built to looser tolerances because they still run well, and the wider the tolerance window, the cheaper they are to make or subcontract.

To me, a machine gun is simply a machine to empty magazines. A semi is akin to a hacksaw; an MG is a Sawzall. They accomplish the same thing, but one does it significantly faster than the other. And as alike they are in goals, they are just as different in function.

OK, I'll shut up now .... again, good luck!