I've got an sp1 carbine and have yet to find any manual that explains how to zero a 16" barrel using the A1 sites.  I did find a Bushmaster manual that somewhat explained the A1 differences but it didnt help too much.   It basically said the zero would got out to 250yds instead of 300 for the A2.

Should I use the unmarked rear aperature to zero at 25yds like the 20" barrels?  And at what point would I want to use the marked one?  I plan on using 50-55gr rounds only as it's got the 1-12" twist.  

You use the same procedure to zero a 16" as you would a 20".

Use the small aperture to zero.

What distance you zero at will determine the distance at which the bullet trajectory will meet the line of sight plane. 50 meter zero is the current hot trend. You can zero at 25 meters and it will work also, just means the trajectory of the bullet will cross the sight plane about 50 to 75 meters sooner.

40 through 55 grain is fine, just make sure you zero with the cartridge you intend to shoot the most.

Zeroing an A1 is easier because you have no elevation to jack with at the rear sight.

you can use an unfired FMJ cartridge to adjust your sights.

On an SP1 (just like an M16A1) both apertures are small - the difference is the height of the apertures.

The standard M16A1 zeroing procedure (use the 'L' marked aperture at 25M to zero, then flip to the unmarked aperture) doesn't quite work as the sight radius changes offset between the 'L' and unmarked apertures.

The best solution is to use the unmarked aperture and zero it at 50yards.  This will produce a nice flat trajectory where the bullet stays within 2.5" of the point of aim from 0 to 230yards or so.  The aperture marked 'L' would be used at longer ranges (400M or so), but I wouldn't worry about that with a carbine.

Sorry I was thinking of the A2. My bad.

Thanks guys for the input.  Since it's a carbine, Im not looking to shoot beyond 200yds so I will have to stick with the unmarked aperature.  Being that it is a carbine, do the adjustments(as far as windage and elevation) remain the same as the 20".   In other words, does one click on the 20" equate to one click on the 16" whether it be for windage or elevation?

i have a couple of A1 style ARs, and i changed them over to the A2 aperatures.  i like the large hole and zeroed it at 25m and can make soda bottles dance out to 100.  for less than $10 i think it really improves the A1 sight set-up.

edit: spelling

Quoted: ..  In other words, does one click on the 20" equate to one click on the 16" whether it be for windage or elevation?

No, that changes as well.

For more information consult the chart at  the bottom of groups.msn.com/TheMarylandAR15ShootersSite/zeroingmethods.msnw

In general the carbine's adjustments will be roughly 50% more than that of the rifle.

Thanks Forest for the link.  From what I read, I should be zeroing at 50yds instead of 25yds.  Will have to try that next time at the range.  In the article it stated that this type of zeroing will produce a 2" margin from 8m to 220m.  That seems perfect for me.