I have a 12-15 converted to centerfire with a .357 Maximum chamber, set up for breechseating as a Schuetzen rifle.  I'm building another as a .30 cal. 32-20 for my wife, also as a Schuetzen.  I have a Vickers that started life as a .22 l.r., and is now a Hornet with a Belgian proofed centerfire breechblock.

David
Montezuma, IA

I have 30-20, 22 hornet (both in 12/15's), 310 cadet and 38-40.  The last one is a bit big for the barrel shank though.
30-20 (32-20, but with 30-cal barrel) is probably the most versatile, but 22 Hornet is also lots of fun...
They do all have 1 thing in common :  They are accurate!
I'm still thinking of building a .310 match ifle though, and a 30-30 Wesson BP one for midrange work, so many projects...

Forgot to add that George just got another one from 
Thad Scott in 44-40. Cute little thing. Joe

The cadets have been converted into many, many small game cartridges here. .222 rimmed was very popular, .22LR, .22 hornet, .17 Hornet, .357 mag, .32-20 and .310 can still be located fairly easily. Cheap too.

I have a little cadet in .22k hornet. I love the little rifle. Its main use is shooting Rabbits, Fox and Crow here on the ranch. I was chasing a .218 bee improved, but this came up and the price was too cheap to resist. I don`t regret it, as it is relatively quiet, little recoil, standard hornet ammo is available in about any gun shop and cases are plentiful and cheap to buy. I needed something to bridge the gap between the .22 LR and my .243 for small game and this fits the bill perfectly.

Cam...

I have a small framed Belgium Martini rook rifle. It's chambered for the 230 Morris Long, which was the world's first centerfire .22 caliber. 
I form the little Morris from .22 Hornet cases, and it runs at velocities around 1,000-1,200 fps. I love plinking with it, but I've also taken a few small critters too.
This is the Morris next to a .25-20 WCF.
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I have two cadets, one in .310 Cadet and the other in 297/230 Morris long, plus a Mod 12 converted to .22 Hornet.  Of the three, the Morris is definitely the most interesting. Easy to see what inspired it as the training round for the 577/450.
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I have two centerfires small frames....one in .357 mag and one that is just getting the finishing touches....in .223 remington with a Snapp extractor.  I'll post pics when it is done.

PS: I'm new to the forum, but a regular and moderator on the Martini Henry Forum over on British Militaria Forums.  Nice to see a few of the regulars here!