Military Guns and Ammunition

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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.

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subsonic 7mm   General Military Discussion

Started 4-May by smg762; 773 views.

From: smg762


one of my pet designs involves a 300blackout which is necked down to 7mm. as well as subsonics, it takes a sabot round with a .17cal bullet.

the case is wider than a 300blackout because a 300black sabot round would only have about 1100ft lbs and i want more like 1500.

anyway, the main feature is that the 7mm subsonic would have a weight of 270 grains, in order to have more energy than a subsonic 300.

also, the neck almost completely covers the bullet/sabot, to better protect the sabot. Therefore, the subsonic bullet must be flat nosed - i dont want the complications of a fancy CT round.

my question is, could a 7mm 270grain bullet actually be stabilised, given that the muzzle energy would be very low.

also, could this unusual bullet share the same twist rate as the much more powerful sabot loading?  assuming a 50grain bullet and 15 grain sabot.

apparently the 300blackout CBJ round can share barrels with subsonics, 'out of the box'. But thats not a 270grain bullet....

also, could you have a muzzle device which slows or stops the sabot, to avoid safety issues?

  • Edited 04 May 2023 9:39  by  smg762
In reply toRe: msg 1
Mr. T (MrT4)

From: Mr. T (MrT4)


Why ? With subsonics its literaly the other way around go for as large caliber you can stuff in the case.  Off the shellf 7mm bullets stop at sub 190grain


From: smg762


i wanted 7mm because its more likely to be accurate as a sabot round - in short barrels.  im just wondering if it will stabilise - and share rifling with the sabot

also, what are your thoughts on this round as a military cartridge - assuming the design works well with subsonics AND sabots

would this be a tactical advantage for troops, being able to switch to subs so they can be more stealthy

  • Edited 04 May 2023 10:07  by  smg762
In reply toRe: msg 3

From: farmplinker2


If you want to switch between subs and supers, a .300 BO will do it with less work, and is already available.

In reply toRe: msg 3
Mr. T (MrT4)

From: Mr. T (MrT4)


My small frame 'long range' wildcat most you can squeeze into AR15magazine and onto a 7.62x39 bolt face.

6 SPARC compared to  6BR 

6 SPARC chamber green , Hornady 6ARC blue


From: Gr1ff1th


Hello Mr.T, what is the concept behind this wildcat, and what advantages does SPARC offer over ARC, thanks 

Mr. T (MrT4)

From: Mr. T (MrT4)


More speed with great ES&SD , something that would squeeze max from my bolt action rifle design that feeds from AR15 mags

In the competitive world, 6ARC turned out to be a bit of a dud , to small to push the 105grain bullets at decent speeds.

That was well understood before ARC even existed when 6AR  came to the scene and 6ARC is nearest clone. 6AR spawned wildcats with more case capacity in the form of 6ARturbo,6ARTurbo40 ,6FatRat  my wildcat is close the these with shoulders blown forward and 40° shoulder angle plus non tapered case .

6AR and 6ARTurbo40 

Note there are cartridges with more case volume for AR15 ,like 6HAGAR build on base of .30Rem but this is how they are loaded for feeding from the magazine