I'm not sure what a FN 91 is, maybe you're referring to a FN FAL? The H&K 91 is a 7.62 semi-auto battle rifle, derived from the H&K G3, full auto rifle. The PTR 91 is a clone of the H&K 91. I acquired mine back in the late 70's, and I hunted white-tail deer with it for many years, using a five round magazine. It's an older design, but extremely reliable, and it is on the heavy side. It's a weapon that can tolerate rough conditions, and it always goes boom when the booger hook activates the bang switch. Due to the roller lock action, recoil is less of an issue than with other 7.62 rifles. Magazines are relatively inexpensive too. It's always interesting to fire the H&K 91 after firing an AR-15, as it's much louder, and just feels more authoritative. In an Alamo situation, the K&K 91 is the weapon that I'd prefer to be using, as the 7.62 has significant stopping power, and I wouldn't be humping it.
The AR-10 has more accessories available, but the H&K 91/PTR 91 now has trigger, stock, and hand guard upgrades that weren't available back when I bought my firearm. The AR-10 is a lighter weapon, and most folks are familiar with the action and how it functions.
