Hello,
Did you take a look at the 44 magnum?
It fits in the bolt head.
I dont think that it will cost you much more to rebarel in that caliber than reboring an old enfield barrel.
Make sure your get a barrel in 44 mag (.429) with a 1 in 16 or better 1 in 14" twist.
you can used either desert eagle pistol mags or modified M1 carbine mags.
And hornady 300gr xtp bullet should do a marvel in such rig.
take a look at the following article, middle of the page:
http://home.xtra.co.nz/hosts/suppress/news.htm
Good luck.
NIC