The original SP1 buttstock is slightly shorter than a newer A2 buttstock, ~ 10" as opposed to ~ 10.5".

So......

A2 buttstock buffers are to long (bolt won't go back far enough to grab the bolt catch). Carbine buffers will work but the "H" ones weigh quite a bit less than the 5.15 oz normally associated with rifle buffers.

I found a FAQ on rec.guns that said that Colt used a lightweight buffer on the original SP1 but what's "lightweight"?

What about using an H2 or H3 carbine buffer (right now I can't find an H3 buffer)? These weights would put me right around rifle buffer weights (H2 just under, H3 above).

Thanks all.

ETA, at least I *think* the carbine buffer will work. It does allow the bolt to lock open, but what problems might crop up if the shorter buffer allows the bolt to move to far back? This is an area of AR "mechanics" that I'm a bit unsure of. Does the buffer actually stop the bolts movement? If so, then the carbine buffers might not work for actual firing.

Just use your regular rifle buffer tube and rifle buffer and spring.  The difference between the A1 and the A2 is the A2 has a spacer.  To mount an A1 stock just omit the spacer.  And shuuzzamm!  Hope this helps.

Regards,
PatG.

Yeah, I jumped to a boneheaded conclusion. The actual buffer tubes on this rifle and an A2 I own are the same length so I've got something else going on. Because a rifle buffer will *not* let the bolt move back far enough to catch the bolt latch.

Back to the drawing board.

If you are going from an A2 to A1 stock, make sure you use the shorter A1 buttstock screw. The A1 & A2 both use the same buffer, spring, and buffer tube. The difference in stock length is made up with a spacer, and longer stock screw. The SP1 is the same as an A1 stock, BTW.

Don't use a Carbine length buffer in your Rifle. Your lower may get damaged by the key on the top of the bolt carrier, as it slams into the rear of the lower reciever.

Quoted: If you are going from an A2 to A1 stock, make sure you use the shorter A1 buttstock screw.

VAPORTRAIL, this is exactly what was wrong, I ordered a regular A2 buttstock screw. Damn the details!! I actually found the problem just before you posted. After measuring the tubes and finding out they were the same length, I'm sitting there and got to thinking that maybe something was in the bottom of the tube. Aimed a flashlight down there and the screw was staring me in the face. Sigh.

Thanks all.