Thread: 300 Blackout barrel length

Hi All,

What length barrels are people running in their 300 BO bolt rifles? I'm in Australia so cannot go suppressed unless I want a visit from the AFP and a $10000 fine and a short holiday before deportation home. Basis of the build is a Brno fox currently in 17 Mach iv and I am looking to set it up like Wingman's switch barrel cz527 I also want to have a play with a 6x45 or 250 myra in the future. Use of the rifle is hunting fallow deer with a relatively compact rifle mostly in bush blocks with the range not over 200 yards. I have other rifles for the far off stuff.

Mines a 12” 1:7 custom and a couple of mate use 16” standard factory Rem 700.

Mines 12.5 inches

Subs only thru a suppressor
Make a 007 suppressor out of an old maglight torch
I mean maybe don't but 300blk isn't worth pursuing without a suppressor

Originally Posted by Bill999

Mines 12.5 inches

Subs only thru a suppressor
Make a 007 suppressor out of an old maglight torch
I mean maybe don't but 300blk isn't worth pursuing without a suppressor

Know a bloke 40km up the road who got a visit from 15 blokes in black pyjamas for his non mag lite, maglites.
I hunt with a bow too so up close isn't a problem.

I mean without suppressors you are as well served by a 308 or 30-30
300blk is not special without a suppressor

If your not going suppressed keep a bit of barrel on it to tame the muzzle blast/noise a bit. 12 is LOUD without a can. 16 to 18 would be a good compromise still be handy as.

I agree with Bill though, the cartridge is really designed to be suppressed. Way more versatile options in mini action now. 6.5 grendel springs to mind, finished at 16 it would be a mint fallow gun

The only real reasons to go 300blk are either to suppress is or to shoot it in an AR.
If suppressing is not an option you may aswell go for 6.5 Grendel or one of the other cartridges you mentioned.

As far as barrel length, what ever you feel works for you.
18" is a nice mid length, for a bush only gun under less than 20" is probably recomended

You guys have said a blackout needs to suppressor to be better/effective. How so? Is that for subs only or for supersonic as well..?

I have one and use a suppressor. Mainly with supersonic ammo. I like the cartridge.
@Beetroot @Bill999

Thanks for any input.


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Suppressed or unsuppressed, the ballistics are still the same obviously.

The point is if you can't suppress it and do subsonic loads, it's going to bark like a 308 with supers, so why not just buy a 308 which has better ballistics?

16" Ruger American Ranch here with a DPT can on it and a Nikon Monarch 2-8x32. So very light as well. Bowls over Fallow really well (I use the American Gunner 125gr supers).

I still haven't put a single super thru my 300blk
Unsuppressed 300 blk with subsonics would still be better than supers if you needed a bail gun to save your dogs hearing
300blk goes well with supers from short barrels but its deafening
Can you put whistle tip (blast mitigation device) on your rifle?

Originally Posted by Pommy

Suppressed or unsuppressed, the ballistics are still the same obviously.

The point is if you can't suppress it and do subsonic loads, it's going to bark like a 308 with supers, so why not just buy a 308 which has better ballistics?

Agreed. Or choose a short action cartridge with better ballistics.

Its not that 300blk is ineffective without a suppressor, its that 300blk shines because its easy to get subsonic loads for and gives good perfomance from very short barrels (which usually accompanies a suppressor).

While supersonic loads give good ballistics , there are cartridges that perform much better in the same short action.
300blk may have advantage of factory ammo.

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.