My attempts at a subsonic 450BM load

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My attempts at a subsonic 450BM load

Postby mcb » Tue Jul 25, 2017 8:30 pm

As I alluded to in the Barnes thread I have been playing with working up a subsonic 450BM load. I don't have a suppressor yet but I was able to borrow one for a small part of my testing near the end and am seriously looking at getting one for my 450BM.

First the bullet. I started with Remington 405gr Soft Pointed bullet. This is a .458 diameter bullet but one that should work fairly well at subsonic velocities. In the original factory ammo Remington only loads this to ~1300fps so we will not be too much slower in a subsonic load.

I sized the bullet using Lee bullet sizing dies, a good closed arch RCBS press and a generous coating of Lee Liquid Alox . First I ran the bullets through a .454 diameter die, as I did not want to over stress the press trying to size a jacked bullet that much in one go. Next I ran it through the .451 dies. I figured it would spring back some and I was right. The final sized bullets measured between .4515 and .4522. That seem like it would work nicely.

For a first test I created a few dummy cartridges with the bullet seating to max OAL of 2.260 inch. This seated the bullet very nicely to just shy of the top of the cannelure. Crimped it to .474-.475. I then hand cycled these dummies through my rifle to make sure they would feed properly from the magazine and to make sure I was not jamming the bullet against the rifling. Hand cycling looked promising.

Image

The first powder I tried was IMR Trailboss. This was a no-brainer and fairly simple to work up. According to IMR/Hodgdon. Max charge for a modern rifle case is a 100% full case with no compression. Basically you can't over charge with Trailboss assuming you don't compress the powder. Compressing the powder can be dangerous as you break the powder grains effecting a higher than normal burn rate and then you can get into trouble. I set my calipers at 2.260 inches and laid the head of a case against one jaw and the nose of the bullet against the other jaw along side the case. Marked the case carefully and then cut the case off at that length. That volume became my max charge as the would fill the case to the bottom of the bullet. With Remington Brass my max charge weighed 13.7 gr. IMR indicated you could back off to 70% if you liked. Quickloads prediction (optimistic as usual) indicated I did not want to back off the full 70%.

So my first two loads where as follows:

450BM Subsonic #1
Bullet: Rem 405gr SP (resized to .452)
Case: Once fired Remington
Primer: Winchester Small Rifle
OAL: 2.260 inch
Crimp: .475 inch
Powder: IMR Trailboss

12.7 gr
Average: 794 fps
Std Dev: 29.0 (5-shots)

13.7 gr (MAX)
Average: 908 fps
Std Dev: 28.9 (3-shots)

Tested in my factory 20-inch Bushmaster barrel, using a buddies Magnetospeed instead of my Shooting Chrony for the velocity. I have to say I was sort of disappointed here I was hoping to get closer to 1050 fps. It was no where even close to cycling the gun. At best it would extract the case just far enough that the case would hold the bolt open with the mouth of the case on the entrance to the chamber still inside the barrel extension.

So with a little more research and playing with Quickloads I fell upon Vihtavuori N32C Tin Star. I love Vihtavouri powders in my pistol reloading but they are spendy and have become hard to find, but I manage to find a pound of Tin Star online and this turned out to be a pretty good powder for this bullet gong subsonic. Quickloads also was a bit more accurate with this powder but still optimistic.

450BM Subsonic #2
Bullet: Rem 405gr SP (resized to .452)
Case: Once fired Remington
Primer: Winchester Small Rifle
OAL: 2.260 inch
Crimp: .475 inch
Powder: Vihtavuori N32C Tin Star

13.5 gr
Average: 856 fps
Std Dev: 20.3 (4-shots)

14.5 gr
Average: 927 fps
Std Dev: 7.4 (5-shots)

15.5 gr
Average: 979 fps
Std Dev: 8.8 (5-shots)

16.5 gr*
Average: 1056 fps
Std Dev: 10.0 (5-shots)

*Quickloads predict this load is probably above max SAAMI pressure for 450BM but not by much 1,000-2000psi. Work up to it slowly and use at your own risk please. I wish I could get real pressure data but I not sure I ready for that yet.

That said the N32C power produced very consistent loads, I was very happy with that and the 16.5 gr was right where I wanted to be velocity wise. It would eject the empties cases but not pick up the next round in the magazine when fired un-suppressed. BUT with a suppressor on the gun it cycled quite nicely and would lock back on the last shot. It was also nice and quiet, you could hear the bolt cycling over the report of the bullet exiting. I did not get to put very many rounds through the suppressor that trip, but enough to really get me excited.

This past weekend I tried a five shot group at 100 yards not suppressed. I had to use the 11 MOA hold point and the bullets still landed about 6 inches below that point of aim but they did group well and where still stable, punching nice round holes through the target. The group size measured about 2.4 inches.

Image
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Re: My attempts at a subsonic 450BM load

Postby Hoot » Wed Jul 26, 2017 7:49 am

Did switching to the VV powder get the rig to cycle normally? TrailBoss has a long history on this forum of not fully cycling. Do you have a Hornady Overall Length Gauge (formerly Stony Point) and a 450b test case? Heavy, Stubby bullets have a tendency to hit if not jam into the rifling when seated at or near full mag length of 2.26. Slow, Heavy rounds may benefit from a little jump before engaging the lands, in the interest of accuracy. I have zero experience with any loads in this caliber concentrating on subsonic performance however.

QuickLoad Tip: In this caliber, You will get a better model of the chamber pressure if you set the Start Pressure parameter to zero. Otherwise, it tends to over-predict the pressure and velocity.

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
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Re: My attempts at a subsonic 450BM load

Postby mcb » Wed Jul 26, 2017 8:31 am

Hoot wrote:Did switching to the VV powder get the rig to cycle normally? TrailBoss has a long history on this forum of not fully cycling. Do you have a Hornady Overall Length Gauge (formerly Stony Point) and a 450b test case? Heavy, Stubby bullets have a tendency to hit if not jam into the rifling when seated at or near full mag length of 2.26. Slow, Heavy rounds may benefit from a little jump before engaging the lands, in the interest of accuracy. I have zero experience with any loads in this caliber concentrating on subsonic performance however.

QuickLoad Tip: In this caliber, You will get a better model of the chamber pressure if you set the Start Pressure parameter to zero. Otherwise, it tends to over-predict the pressure and velocity.

Hoot


With the 16.5gr N32C load it would sometimes eject but not pick up the next round when fired un-suppressed. With the suppressor attached it cycled well and locks back on the last round.
Ultimately I would like to do a suppressor specific build like many 300 BLK rifles and put a pistol length gas system on a 16-inch 450BM barrel. Drill the gas port to run factory ammo reliably and I believe it would then run subsonic ammo with or without a suppressor reliably also. A suppressor on a 20-inch barrel is pretty unwieldy if but really quiet.

With the Remington 405gr bullet. The bullet's nose if not full diameter. The bullet (after sizing) is .452 up to about .12 inch beyond the forward cannelure. The rest of the nose is .445 or smaller so it is basically is a land riding nose. I confirmed this with my dummy rounds making sure I was not leaving marks from the rifling on the bullet before I proceeded with my first loads.

Thanks for the tip on Quickloads. I will see if that helps with these subsonic loads.
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Re: My attempts at a subsonic 450BM load

Postby plant_one » Wed Jul 26, 2017 8:40 am

trailboss just doesnt prodoce enough gas volume to cycle an ar. i dont think the caliber matters.
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