The front sight base, if not included, needs to be installed by a qualified gunsmith or a machinist
with a good understanding of firearms.   No exceptions.    It can be obtained from Fulton Armory or any of several other AR makers.   Bushmaster won't sell you one, though.  They don't sell parts that require gunsmithing to install.

Accuracy potential can be excellent.  One inch groups at 100 yards, maybe a bit better, with good ammo and better shooting skills.  

I have an SP1 barrel on my 2nd rifle ,and it has 1 in 7 twist, so it's good for even the heaviest bullets.

1 in 12 will be good for up to about 55 grain bullets.   1 in 10 will take somewhat heavier bullets,  1 in 9 will take heavier bullets stilll,  and it really takes a 1 in 7 to handle the heaviest bullets like 77 gr. Black Hills ammo.

The faster the twist, the heavier the bullets you can shoot and still get them to fly right without tumbling in flight.

An SP1 is Colt's initial AR-15 offering to the public.   They date back as far as the late 60s but they were made for a number of years.  They're the granddaddy of all AR's.


CJ

An SP1 is Colt's first AR-15 variant sold to the public.

It's very similar to the M16A1 except it's semiautomatic.

It has the shorter A1 stock, the triangular A1 handguards, the A1 type upper receiver without a forward assist or a brass deflector, has no magwell fence on the lower receiver, and has the thin, light pencil barrel like you've been offered.    Nothing wrong with it, in fact you'll appreciate how light the rifle can be with that light barrel.

This is an SP1:

It IS a classic look, that's for sure.

I know SOMEBODY sells A1 furniture,  but I don't know who that somebody is.
I'm sure someone else here knows and will tell you, though.

If you want authenticity,  make sure the upper receiver is an A1 style,  like this:

(Click on the links as some won't display here.)

www.bushmaster.com/shopping/uppers/Images/8448524.jpg

(But one without the forward assist or brass deflector is more period correct for an SP1 clone,
however you may have to find that on the used market.)

Compare to an A2 upper:

www.bushmaster.com/shopping/uppers/Images/a2ur.jpg

Note the very different sight assembly.  It's the major difference.


Next, the tough part:  Trying to find a "correct" slabsided LOWER receiver!

Here's a fine pic of a true slabsided lower, but this one is one you can't have as it's from
an M16.  (Full auto)  

home.cfl.rr.com/c3stuff/norrell/norrell-right.jpg

Compare the sides of it to this later model receiver, with the fence around the
mag release button, and with other updates:

www.bushmaster.com/le/images/lower_le.jpg


Good luck!

CJ