Re: AR Trigger Groups?
Actually, both answers are correct....and wrong.
Ar15 and AR10/LR308/etc. trigger pin holes and relationships ARE the same so they ALL will FIT (but maybe not function properly).
In other words, a STOCK, so-called mil-spec hammer, trigger, & disconnect will work in either gun, some manufacturers use a heavier hammer spring (DPMS does NOT, all parts are the same as their A15).
AFTERMARKET triggers are all different, I like the Geissele's as they use FULL power springs and have the fastest locktime on the market, I have them in both Ar15's and 10's, same part # for either gun.
Some other brands have different part numbers for the different guns and this is the reason.........
Most other aftermarket AR15 triggers use a lighter trigger spring and lighter hammer to achieve a lighter pull and lock time, this sometimes causes problems with the heavier firing pin weight of the AR10/LR308/etc. (Armalite also has a return spring on the FP, which takes even more power to hit).
Sooo...they make another part number with a heavier spring and/or hammer to overcome the inertia of the heavier FP.
Anyway, just get a Geissele and you'll never regret it.
If it's just a play gun and you want to go cheap, buy an RRA 2 stage for $85, (they use full power springs) but don't use one on a gun you may need to trust your life with as sometime (may be 500 or 8000 rounds) it will fail, as will most of the other aftermarket triggers.
If the cheaper trigger doesn't hit hard enough, just stick a Wolff HEAVY trigger spring (from Brownells) in it.
HTH