Quoted: Really I'm trying to maximize my tax stamps... |
Most 9mm cans can handle .300 Whisper. If you get a 9mm can, you can use it on multiple 9mm platforms as well as a .300 Whisper upper (the Trident sounds pretty good on my T/C Contender Whisper). I believe the majority of 9mm cans could easily handle supersonic .300 Whisper loads (it would be about like shooting a .357 through the can). If you slip up and fire a full-powered 7.62x39 round through a lightweight 9mm can, I don't know if the can would take it or not. Using the Whisper gives you a better margin of error.
If you are maximizing use (meaning, buying the minimum number of cans to be used on the maximum number of guns) the answer would depend on what you want to shoot more of - pistol or rifle cartridges.
I've got a small .223 can (too loud on an AR, but sounds good on a 23" Contender). I've been thinking of getting a .30 caliber can. It would primarily be used for the .300 Whisper, but could do double-duty on a 5.56 AR15. The tradeoff is that it would be larger & heavier than a dedicated .223 can.
The other issue, as you noted, is case prep for the .300 Whisper. If you are handloading, there is no special case prep for the 7.62x39. Forming Whisper brass takes a bit more work (as I'm discovering).
Factory ammo isn't really an issue. Both are semi-custom propositions, and both cost in excess of $1 per round.
The Whisper gets the nod for reliable mags. 7.62 mags can be difficult to find, and may or may not work when you do. Again, up to you.
Not sure what the standard twist rate is on 7.62x39. You can get a Whisper barrel in a 1:8 twist, which gives you the option of shooting bullets up to 240 gr. Keep in mind that shooting a subsonic 154 gr. .30 caliber bullet is only marginally better than a 147 gr. 9mm. (with that in mind, just build a shorty 9mm upper and put a 9mm can on it. Again, the Trident sounds really good here - but weren't you planning on getting one of those anyway?)
With the larger bore diameter of the 7.62x39, I'm not sure if heavy bullets are even available. Unless you get a barrel custom made with the .308 bore, and then (since you're into a custom barrel) you can get whatever twist you want.
Just a few things to consider...