I never built rifle corps but I have assembled too many 3 point connection models than I would care to admit.
Normally, my approach is to clean all the pieces from flash, then lay them out, dry fit each, dry fit again until you find the best combination. Once each "best combination" is met for each body, you should have all of your bodies with the right arms / guns.
If this makes you nervous, go to home depot and get some acetone and the right safety equipment as that's the easiest way to fix pewter mis-glues.