Quote Originally Posted by sclg View Post
Short of re-proofing it is there any way to safely explore this whole idea?
Not unless you are attracted to the idea of possible maiming.

How can you determine if safe limits are being reached?
It's safe until it isn't.

Would it be feasible to assume that if MVs are kept within reason (he wants to get to 750fps) then peak pressures are not excessive?
No!
Standard ballistics curves show you the chamber pressure against distance travelled down the barrel. The area under that curve is the bullet velocity. Hence if you want more velocity you need a longer barrel or a higher chamber pressure. The problem is that a) he has no idea what peak pressure he's generating and b) he has no idea what peak pressure is excessive.

There are far too many unknowns.
S
You can take it to the bank that NEITHER UK Proof House would proof-test-fire a BP handgun for ANY nitro load. Apart from the fact that it would be illegal, that is.

A firearm made for a nitro load is deemed fit for use with BP - indeed, many replicas of rifles can shoot both types of cartridge safely - Trapdoor Springfields [with lead only], Sharps, Smith, Rolling blocks, High Walls et al, ALL with lead projectiles, but not the other way around.