Will an SP1 upper mate with a RRA lower reciever?

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not without offset pins.

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Quoted: not without offset pins.

Offset pins are for use when you're matching a current UPPER to an SP1 LOWER.  When it's putting an SP1 UPPER on a curent lower, the only practical solution is to mount an adapter sleeve into the front hole of the upper.

DPMS makes such a sleeve.  Only costs a couple of bucks, but it is a bit of a bear to properly install. The hole in the sleeve is off center, so you can adjust the position of the thing to give you an even positioning of the upper to the lower.  You start with the thin side of the wall straight up in position, and adjust it slightly back or forward, to get an even position along the upper/lower joint. Once the right spot is found, the sleeve can be Loctited or epoxied into that postion, and you've created a small hole lower.

So Colt SP1s and current Rock River Arms guns use different sized pins?

Quoted: So Colt SP1s and current Rock River Arms guns use different sized pins?

Colt SP1s use differnt size pins than every other AR-15 for the most part.

Quoted: Quoted: not without offset pins. Offset pins are for use when you're matching a current UPPER to an SP1 LOWER.  When it's putting an SP1 UPPER on a curent lower, the only practical solution is to mount an adapter sleeve into the front hole of the upper. DPMS makes such a sleeve.  Only costs a couple of bucks, but it is a bit of a bear to properly install. The hole in the sleeve is off center, so you can adjust the position of the thing to give you an even positioning of the upper to the lower.  You start with the thin side of the wall straight up in position, and adjust it slightly back or forward, to get an even position along the upper/lower joint. Once the right spot is found, the sleeve can be Loctited or epoxied into that postion, and you've created a small hole lower.

that's what i meant.

Thanks

In short, don't bother.  If you want an M16A1 look, just buy a surplus A1 upper.