i have had this old sp-1 AR for a long time and now i would like to make it into a flattop reciever but...it has the large screw/bolt-pin.

what if anything can i do to overcome this pin diameter problem??

You can get large to small pin converters, and plenty of people sell large pin uppers.

There is a web site devoted to making your SP-1 look like one of the original M-16 variants that General Curtis LeMay bought for the Air Force. If you are handy with files and such, you can sell your rig for enough to buy something a bit more modern (looking).

Also Mark (Marc) Brown makes a nice little converter where your gunsmith cuts off most of the handle, mills the top of the receiver flat, an puts on an attachement that gives you a picatinny base. Yaba daba do.

LD

You can find flat top uppers for pretty cheap. I bush and redrill the Colts lowers to accept all Milspec uppers. Send me a PM if interested.

Oly used to convert carryhandle uppers to flat top using weaver bases. I have one from 1987.

Rich

The old Sp-1's are gaining in collector value, I would just build a new complete upper and leave the original Colt alone.

thanks all. srry took me so long to get back here as my work schedule is crazy.

im just begining to investigate and read up on the mechanics regarding my project and seems pretty straight forward.

couple of thoughts id like to run here tho. can i simply buy a small hole upper settup complete and ream the front hole .250 out to the .312 diameter or is there not enough meat there to handle the larger hole?

also is there a trick to attaching a new barrel to another upper reciever and linning up so everything works?

i will keep my origanal sp1 upper as is. dont want to modify it nor do i really want to modify the lower as id like to keep this all an orignal sp1 as i like and have a special place for it to remain as is except id like a complete upper to pin on for another fun and different look/function.

the trigger groups i see for sale,,,are they timed already or will i need to monkey with them? i already know about the different pin dia's and have found trigget groups that offer both dia's pin's

thanks again for the interest and input
ted

quote:

what if anything can i do to overcome this pin diameter problem??

Drill it with a 5/16 drill. I've done it. Just bungee the two halves together (rear pin in place and pushed in), clamp the halves gently into a vise (best to use one of those lower "block" things), have someone hold it to reduce vibration and carefully drill the hole with a spankin' new bit. Just do it slowly and you should have no problems. Then get one of these things to fit the 5/16" hole...

http://www.brownells.com/.aspx...BUTTON_RECEIVER_PINS

You can get adapter pins to mate the new upper to your lower, they are not expensive. A new upper receiver runs about $100. A good barrel like a Bushmaster will run around $240 ready to bolt on. A complete bolt w/carrier is around $150. To install the barrel without damaging the receiver requires a few special tools, basically unless you just want to do it yourself, or you are planning on doing more than on, you would be better off buying a complete upper assembly. The trigger parts are normally "drop in" but likely will be a bit creepy without stoning. The Rock River 2 stage match trigger is very good and fairly inexpensive. You should go to AR-15.com and you can learn everything you need to know about Ar's, and there are plenty of vendors with competitive prices there. Good luck.