It may not fit, Ruger peens (dents) the 22/45 receivers on the underside of the front lug recess to tighten them on the frame. This means that typically a 22/45 receiver doesn't fit onto a regular metal frame. However some will fit without any alterations so you just have to give it a try. If it doesn't fit, then you can polish out some of the inner lug area to seat the receiver. Also, Volquartsen is planning on coming out with an aluminum 22/45 frame soon. They are projecting approx. two months before they're ready to ship frames. VQ will announce them on their website when they are available. I haven't heard what the expected price will be for these yet.
I use a diamond burr for the Dremel to radius that dented area to put it on a steel frame. That is the only way to get that fixed sight 4+- inch barrel that they sell on the 22/45 frame onto a steel frame where it belongs. That fixed sight barrel with the flat sides and heavy contour makes the perfect carry gun on the steel lower frame. Mine just happens to be dead on for the sights as to where it hits compared to where the sights are too. I don't know if they all are like that or not. Better to undent the dented bottom of the receiver than to alter your frame. Then you can put the original barrel back on if you want to.
Thanks guys - you answered my first question. I picked up a takeoff MKIII 22/45 barrel/receiver, and then got a MKII lower in parts from my gunsmith. When I try to put the barrel on the grip frame, the tang wont slide into the receiver lug. Now I know how to fix it...
However, once I do get it to fit, then how do you remove the loaded cartridge indicator, as I have a MKII bolt? It looks like the pin is pressed in but there is no way to press it out (at least not apparent to me). Thanks in advance, Tom
It is not pressed in, it is a slip fit. You remove the upper, and using a plastic hammer, "shock" the pin out by tapping the receiver AWAY from the direction your want the pin to move. You are really using the inertia of the pin to extract it with the sharp taps on the bottom of the receiver. Once it comes out enough to get hold of it, just pull it out.