Thread: 300 Blackout barrel length

You need to consider the donor platorm you are working with.
The .222 Brno fox has a shorter mag than the later CZ527 .222.

The Fox had different length mags and bottom metal for all the different cartridges it was offered in, while the CZ 527 only has two different bottom metals, .17 Hornet/.22 Hornet, .222,.223/.300blk and 7.62x39/6.5 Grendel.

The .222 fox mag is quite a lot shorter than the .223 CZ 527 mag. this is the best pic I have to show that. (.222 fox mag, .22 hornet Brno 465 mag and .223 CZ527 mag. Ignore the .22 TCM round)





The modern CZ 527 mags use the same overall dimensions of the mag but have various side stampings for different cal bullet support and rear blocks to suit various cartridge lengths as can be seen clearly on the 527 .17/.22 hornet mag here:




The .300BLK and the 6x45 is best built on the new CZ527 .300blk mag offering which has no bullet guide side pressings to allow for the larger cal bullets and it also handle a longer COAL at 59mm. This however will not fit in your .222 Brno Fox bottom metal. You could swap the trigger group and bottom metal out with a modern 527 one if you could find one but Id say a new rifle would be the easier option.
Both the 6x45 and .300blk need to be seated as long as possible to get the best from the cambering, they both need to make the most of case capacity to get the velocity up (apart from BLK subsonics) and both have longer throats than a .223 and .222 to deal with much heavier bullets.

This is a 6x45 round in a .300BLK mag.. note the lack of sidewall stampings. The 6x45 rounds in a .223 mag constantly got jammed nose down and feed like crap.







.300 BLK rounds will not fit in the Fox .222 mag at all due to these sidewall stampings.
If you are still hell bent on using your Fox for the donor you can modify your mags in the mill by machining out the pressed section which creates a nice window to see how many rounds you have left and collect a bunch of dirt and grit while you are at it.



You should also keep in mind that many bullets you will use in the 6x47 and .300 BLK will have to be seated so far into the case neck that the bullets ogive will be behind the mouth of the case and it will have a massive jump to lands on any std freebore cambering. The trade off in doing so is velocity and accuracy potential.

My advise to you is to sell your .222 Fox with the old .222 barrel if you still have it and purchase the CZ 527 for your switch barrel platform. keep your current barrels as they will still thread into the new 527 and your smith can set up their headspace correctly.
Re: your personal message: I understand your gunsmiths concern and he is acting responsibly as he cant control what you do once the rifle leaves his workshop and doesnt want the responsibility of something going wrong.
His alternate suggestion would work as long as you are consistent with the spanner so that you dont alter the headspace each time you swap them out.
A punched dot/line on the barrel that aligns with a dot/line on the action works well to keep the head space consistent between swaps.
Another option is to drill a 4mm hole in the top of the action into the seated barrel thread and use a pointed grub screw to locate and secure the barrels each time.