Thread: Loading for .300BLK

Some of our Aussie members may have seen some of this .300BLK load development on the Aussie forum but I have done lots more development since then and will add the more recent stuff here too.

A sunny afternoon and a a few hours to experiment I decided to load up some .300 AAC BLK cast bullets that Ive been wanting to test on paper..
I have some 245gr and some 155gr cast bullets that I wanted to try with and without gas checks to see if the rebated "boat tail" like shape or a flat base would make any difference to group size. There is no real need to fit a gas check as they are only loaded at subsonic speeds.

The test platform is a Remington 700 Tactical AAC-SD .300 Black out
16.5" long 1-7" twist M24 type 5R barrel 5/8"x24 tpi thread
MDT LSS (Lightweight Sniper Systems) Chassis
AICS / MDT detachable 10 round magazines
Magpul CTR Carbine Milspec adjustable stock
Magpul CTR/MOE 0.50" Cheek Riser Black (not fitted in pic)
Magpul MOE + AR soft touch duck bill pistol grip
Harris 6-9" Swivel SLM bypod
Hardy custom AAC Gen 4 7075 alloy over barrel suppressor
Custom larger Badger ordnance style bolt knob
Weaver 20MOA Picatinny rail
Fixed 10x40 Bushnell Tactical scope with butler creek flip covers





The rifle will shoot half MOA with 240gr SMK subsonic loads but the projectiles are a little pricey so I save them for hunting rather than range work.
It also shoots 125gr Nosler ballistic tips at 2320fps which group around an inch at 100y.






What I wanted was a cheaper subsonic bullet alternative to use around my property on rabbits and general plinking as it is a rifle I really like to shoot ..

I started off with the 245gr cast bullets from Robert which looks like a “perfect moulds” mould? I loaded them with several powders such as AR2205, AP 50, AP70 and Trail Boss but got the best (I use that word loosely) groups with 8.3gr of Hodgdon H110 both with and without gas checks fitted shooting an average of 1020fps.. this is the result at 100 yards.



Gas check fitted


Without gas check


Fairly average results at best and really inconclusive to whether the gas check makes any difference to ballistics.

Next up I loaded some 155gr Lee Spitzers that I cast and shoot in a custom big bore PCP air rifle. The twist rate at 1-7" is too fast for these lightweights but I thought I give them a try anyway.. I loaded them with 8.5gr of AR2205 to 1050fps and they are VERY quiet!



They are a very low pressure round with that powder and there was a lot of sooty black blow by on the spent case necks..
However these shot far better than the heavy weight bullets and could be fine tuned with a faster pistol powder to bring the pressure up a touch.

No gas check


Gas check fitted



Besides the flyer in non gas checked bullets the group would have been about an inch.
I was very happy to see a group of 3/4" from the gas checked bullets and it is defiantly a contender as a cheap plinking bullet! I will test it further with other powders and at longer ranges very soon. I have a large flock of Canadian geese that frequent a field about 250 yards from my shooting bench so it would be great if these silent lightweights hold stability to that range..

The mould is the 90385 Lee double cavity 7.62x39 .312 " 155gr bullet that I cast in pure lead and size to .308" the listed B.C is .268

I use pure lead for casting my sub bullets as its natural slippery properties and the lack of tin/antimony actually makes for less fouling. As a subsonic bullet it has no need to be hard. I also found sizing them to .308” instead of .309” also got better groups. I found flaring the case mout lightly before seating gave less bullet damage when seating and no shaved lead rings in my throat, I also use a light roll crimp to lock them into the case which gives much better velocity consistencies.




A little more testing with some IMR Trail boss powder and the 155gr bullets over the chrony and had two loads to try, 6.0gr of T/B gets me 995fps and 6.5gr of T/B gets me 1020fps but its an absolute max load and borderline compressed which I find T/B doesnt like that much.. I have my money on the 6.0gr load.
It is a higher pressure load than the AR2205 loads I tested as it shows a flatter primer and no sooty "blow by" on the brass when extracted. I do like how clean T/B burns..

It is a super quiet load and shoots 3/4 moa with the 2205 but I would be very happy if I can match that with the T/B without the sooty blow by low pressure issues of 2205.



I decided to scrap the compressed 6.5gr T/B loads after putting a few more over the chrony, they showed large fluctuations in velocity as I expected they would.. Trail boss does not like compressed loads.

The 6.0gr of T/B felt real good to shoot but the group really fell apart with this powder.. 10 shots in just over 5" on paper!
However the Chrony results were very good with only a 18fps max spread over 10 shots.

I think I will put up with the sooty low pressure load of 2205 as it shoots extremely well!



A very enjoyable few hours on the reloading bench and the front deck.. look out bunnies!

More bullet tests to follow...