I'm wanting to build a slide assembly to convert my stainless Colt 1991A1 into a 10mm. From researching I'll only need a barrel, 40 S&W/10mm slide, slide parts, ejector, and 23 lb recoil spring.

Sound right??? Since Colt doesn't use a ramped barrel, I'm looking at unramped 10mm barrels that should drop in.

I know I can probably buy a RIA 10mm for about the same price, but will be building this over time.

I would cut the frame for a ramped barrel.

+1

might as well keep the gun " as is " and build up or buy a 10mm

.

Bad idea.

The beauty of a built up 1911, is hand fitting the parts

If you're gonna do this, have the frame cut for a ramped barrel,  and have a quality match barrel HAND FITTED

Buy a Delta Elite.

Totally agree...buy one made that way.

The feed ramp angle for a 10mm is different from the feed ramp angle for a .45 ACP.

It is supposed to be the same as the 9mm/.38Super/9x23mm, but my Delta elite is just slightly different.  (I haven't measured anything, it may well be in tolerance, but is noticeably different from my .38 Supers).

If you go to a ramped barrel, this problem goes away.  

In my youthful ignorance I once built a 9mm frame with a .45 ACP slide/barrel.  (OK, I wasn't totally ignorant, but youthfully optimistic).  It did shoot, with hardball, nothing else.  That frame is now a .38 Super/9x23mm Winchester gun, though it also spent time as a 9x19.

Switch calibers are fun, but nothing is certain.  My Delta Elite has been a 10mm, a .40 S&W, 357 SIG, 9x19mm, and a .38 Super.  

Building switch caliber guns may be fun, but financially it is not a wise decision.  I like to have fun with my firearms hobby, and save wise decisions for my investments, however.

No grasshopper, you do not. You want to get another 1911 in 10MM.