The forward assist on the AR-15 has always been controversial. Stoner did not want to add it but was made to by the military. Many say that if a round cannot be chambered, and especially if it cannot be pushed in using the bolt carrier thumb groove, it should be ejected and not jammed into the gun.
There is an interesting discussion at ARFCOM discussing the use of a forward assist if low temperatures freeze the gun’s lubricant.
I was upstate this weekend, temp was 0-6 in the am when I was shooting. My colt 6400 was lightly lubed and I used the fa about 4 times in the process of shooting 120 rounds. I was using federal 55gr ammo. The stops occured after shooting a mag going to check out the target. The lube was a little clumpy, kind of grainy. I know if I used rem oil this problem would of never happened but I didn’t and the fa helped. The lube froze up in the upper, combined with the possible metal contraction caused the bolt not to fully close.
For the record, I tried pushing the bolt forward using my thumb and it worked but the fa was so much easier.
( and on a side note: Alternative Small Arms Lubricants : Using Motor Oil in Extreme Temperature )
[ Many thanks to jdun1911 for emailing me the link. ]