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Not that it matters with the direction this thread is going but you do not have to fire form 30HRT, TAC30 or 30 American cases. Just trim the cases and run them through a die. Other than trimming it is no different than reloading any other case.
The good thing about the 30HRT and TAC 30 is you can used up to 149gr fmjs, seems like I even tested some 155gr but chose the 130gr MK319 for the best performance at 2600fps out of a 16" barrel.
The old 30-30 used mostly 150-170gr round nose or flat nose bullets and those would be unbelievably slow in a 300 Hamr if they could be used in it.

 

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They claim 30-30 velocity but with bullets much lighter. If the bullets were the same weight as a 150gr 30-30 they would be 2-300fps slower.

 

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I was using 1680 and could get 2600 with a 130gr and 16" barrel. Homebrewer was using SW BLK or CFE BLK and was getting 2800 with 130gr out of an 18" barrel.
Wilson and Lehigh Defense claim a 125gr will hit 2440, if that was a 150 like used in most 30-30s it would only get to 2290. I was saying a Hamr is apx 2-300ps slower than a 30-30 if the same weight bullets are used.

 

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Standard extensions.
I shortened the 6.8 case so I could use longer bullets, I've never seen Marty's drawing but the difference in length between my TAC30(1.59) and the 30HRT(1.61") isn't that much. Since the TAC30 can't be done at the moment I would do the 30 HRT rather than the American 30 which I know is full length and will limit what bullets can be used.

 

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Yes mid gas 16" had a .089 or .093 depending on suppressor. .089 with standard can, .093if flow through or no can at all. I have not made any 12.5s but they would normally be .003-.005 larger.