Need Help - Subsonic Rounds Tumbling

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markr6754
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Need Help - Subsonic Rounds Tumbling

Post by markr6754 » Sat Jan 26, 2019 1:58 am

Looking for some guidance leading to resolution of a problem I’m having with my new build 8.5” 300 Pistol. I have a 1:7 twist pistol build from Palmetto State Armory, on an Anderson lower. I am shooting indoors, short range (25 yards max), unsuppressed.

Shooting 147gr Armscor factory and 150gr Hornady Interlock SP over 15.4 gr A1680 @2.106” reloads, supersonic, my rounds make sweet, round pukas in the paper. But, shooting 200gr Sellior & Bellot factory, or 220gr Lapua Scenar L HPBT over 11.6gr A1680 @2.250 reloads, the targets are tearing 1/2” jagged rips in the paper. I thought the 1:7 was best for stabilizing heavy, unsuppressed subsonic rounds, but I’m assuming the subsonic rounds are tumbling, and hitting the target sideways.

Is this my life until I can obtain a suppressor, or is there something that I can do to improve the shooting? Appreciate any ideas.

As a side note, it was very refreshing to have all 20 (10 sub, 10 super) rounds hit the paper, and lock back the bolt. I had to laugh...the one failure to lock the bolt was the last factory 200gr Sellier & Bellot round. Both my 150 gr super loads and my 220gr sub loads had a noticeably more pleasant recoil...after the 2nd box of 147gr Armscor rounds my son was done, and I only had the 10 super and 10 sub rounds since they were my first, ever, rifle reloads. I’ll continue testing, but I could load a lot more of the Hornady 150gr bullets and be satisfied...but where’s the fun in that? I still have 300 BLK and H110 to try out.

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dellet
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Re: Need Help - Subsonic Rounds Tumbling

Post by dellet » Sat Jan 26, 2019 3:01 am

Very first thing to do is verify the twist, make sure it’s 1/7. You can use a cleaning rod with a tight patch. Index the rod and barrel so you can tell one revolution. An 8.5” barrel will have almost exactly 1 full twist.
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markr6754
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Re: Need Help - Subsonic Rounds Tumbling

Post by markr6754 » Sat Jan 26, 2019 3:17 am

dellet wrote:Very first thing to do is verify the twist, make sure it’s 1/7. You can use a cleaning rod with a tight patch. Index the rod and barrel so you can tell one revolution. An 8.5” barrel will have almost exactly 1 full twist.
Thanks, dellet. That’s something that I can easily try. I have to clean up after running 100 rounds through tonight, so I’ll check the twist tomorrow.

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Re: Need Help - Subsonic Rounds Tumbling

Post by SwampDog_13 » Sat Jan 26, 2019 5:32 am

markr6754 wrote: Is this my life until I can obtain a suppressor, or is there something that I can do to improve the shooting? Appreciate any ideas.
Be very careful here, the suppressor will not help with bullet stabilization. If it's tumbling early you could very well ruin your can.

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Re: Need Help - Subsonic Rounds Tumbling

Post by Boolit_Head » Sat Jan 26, 2019 2:45 pm

Ragged round holes could mean the bullet shape is just not cutting a clean hole. Keyholes or a oblong shape indicates tumbling. Could you elaborate on what you are getting?

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Re: Need Help - Subsonic Rounds Tumbling

Post by Sharkbite » Sat Jan 26, 2019 6:26 pm

Boolit_Head wrote:Ragged round holes could mean the bullet shape is just not cutting a clean hole. Keyholes or a oblong shape indicates tumbling. Could you elaborate on what you are getting?
Next time you go to the range, dont shoot at paper tgts. Put up. Sheet of cardboard.

Paper will sometimes tear and appear to have been hit by a tumbling bullet. Cardboard will show exactly the bullets attitude at impact.

This needs to be done BEFORE you mount a can to a new platform. A single unstable round thru the can can destroy it :cry:

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Re: Need Help - Subsonic Rounds Tumbling

Post by BoomerVF14 » Sat Jan 26, 2019 7:12 pm

I'm not a ballistician but did stay at a Holiday Inn Express last night. And I read Litz's book. A 1:7 twist should provide sufficient gyroscopic stability for 300BLK subs at even 'low' muzzle velocities, but we can't be sure without specific MV data. I'm putting Nosler 210gr RDF's through a 9" 1:8 twist at around 1020fps and the math is solid.

Bullets can wobble a little before aerodynamic forces put them "to sleep" and they fly like the tight football spiral we want. How far downrange it takes can vary quite a bit based on atmospherics and bullet construction.

I'd try some shots at 100yds and see what kind of holes they are making... although you probably can't do this at your indoor range. If you can measure MV with a chrony, that should be enough information along with bullet specs and atmoshperic conditions to determine mathematically if the bullet is gyroscopically stable coming out the muzzle.

And like SwampDog said, if you think your projectiles are not stable, do not mount a suppressor until you are sure they are. Try Shark's idea of cardboard backing, and maybe change the type of paper for your target too.

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Re: Need Help - Subsonic Rounds Tumbling

Post by markr6754 » Mon Jan 28, 2019 1:31 am

markr6754 wrote:
dellet wrote:Very first thing to do is verify the twist, make sure it’s 1/7. You can use a cleaning rod with a tight patch. Index the rod and barrel so you can tell one revolution. An 8.5” barrel will have almost exactly 1 full twist.
Thanks, dellet. That’s something that I can easily try. I have to clean up after running 100 rounds through tonight, so I’ll check the twist tomorrow.
Well, that didn’t work. I tried to determine the barrel twist...but all that happened was the brush kept twisting loose of the cleaning rod. I couldn’t get that rod to turn no matter how tight I made the connections...yet I can easily rotate the rod in my fingertips. I don’t have a 30 cal cleaning jag, so I put a patch on a new, 30 cal bronze brush. Patch or no patch, the brush head just unscrews itself.
Will need to pickup an actual cleaning jag and try again. Well, at least my barrel is super clean.

I never realized how tiny the land and grooves are in a rifle (pistol length) barrel...or at least my barrel. All of my handguns have massive lands compared to my 8.5” and 10.5” .300AAC Blackout barrels.

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Re: Need Help - Subsonic Rounds Tumbling

Post by markr6754 » Mon Jan 28, 2019 1:39 am

Boolit_Head wrote:Ragged round holes could mean the bullet shape is just not cutting a clean hole. Keyholes or a oblong shape indicates tumbling. Could you elaborate on what you are getting?
The holes are oblong...there’s not even a keyhole appearance...the paper looks like I whacked it with a ruler. I used the same target as the 150gr supers, which make a perfect hole with no tearing of the paper. Front and back are very clean. With the subs, the paper looks like the bullet hit sideways, with a 1” hanging chad visible, and no semblance of a round puka.

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Re: Need Help - Subsonic Rounds Tumbling

Post by Dolomite_Supafly » Mon Jan 28, 2019 6:39 am

markr6754 wrote:
Boolit_Head wrote:Ragged round holes could mean the bullet shape is just not cutting a clean hole. Keyholes or a oblong shape indicates tumbling. Could you elaborate on what you are getting?
The holes are oblong...there’s not even a keyhole appearance...the paper looks like I whacked it with a ruler. I used the same target as the 150gr supers, which make a perfect hole with no tearing of the paper. Front and back are very clean. With the subs, the paper looks like the bullet hit sideways, with a 1” hanging chad visible, and no semblance of a round puka.
Supersonic bullets are going to make clean holes because of the speed. Slow bullets down and sometimes they can tear through the target leaving chaff. I quit using copy paper because of it. If I am shooting paper I use resume paper, seems to not tear at all.
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