I've just done it on my standard Ducati monster S4 cans, not as short as 300mm but the principles the same. Here's the results...
I made my cut right through, about 100mm in, just used a standard hacksaw it was pretty easy. Knocked the innards out a bit with a rubber mallet and cut through them close to the end caps.
There's a huge amount of baffling in these cans, heavy as well..
Ordered some 50mm perforated tube and packing from
Welcome to AA16 Road n Race Supplies, (about 45 quid total) top bloke, recommended. Rearrange these parts into a well know can...
Put the perf tube in, I didn't secure it, it just sits over the pipe on the 'in' end and I swaged the other end a bit to sit up tight against the 'out' end. Wrapped the tube in the packing and pop-rivetted the end caps back on. Before and after..
Quite pleased with the end result, only tools I needed were a hacksaw, mallet, file and pop-rivet gun. And a lot of elbow grease! Here's the shorty up against an original..
It's a *lot* lighter than the standard. Just gotta do the other one and I'll let you know what it sounds like.