[Scorch]

Sounds like you are talking about a rimless 45 cal version of a 444 Marlin. It can certainly be done, but it will cause you some trouble.

1- Rifles generally do not like cases that headspace on the mouth. The longer cases do not headspace well and can cause issues.

2- Rifle chambers like some taper to them to ease extraction. About .010" of taper. More later.

3- At higher pressures, recoil and bullet backout will become an issue because of the taper crimp. 45 Win Mag rifles were produced, I think they are cool, but I am not sure if they had issues with this.

4- The biggest issue is that brass cannot be made easily from 30-06, you will wind up with a rebated rim case. 45 ACP base diameter is .480" (versus .470" for 30-06/308), and that leaves the brass very thin at the case mouth (.005"-.007" in order to give the needed case taper). You are talking about higher pressure, so you need thicker brass at the case mouth, maybe .012", which will make the case mouth .475" minimum, and your base diameter is .470", so you wil be bulging the brass out to fit the chamber. Problem there, but not insurmountable.

5- For fireforming, seat a lead bullet out far enough so that it engages the lands, that will keep it from being driven forward when the firing pin hits it. Old wildcatters' trick.

Hey, have fun experimenting!