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View Full Version : 55gr XM193 & 62gr XM855 - 1:9" Twist


ExtremeX
04-23-2012, 9:34 PM
Ill start this thread by saying I understand this stuff isn't match ammo, and I am also an experienced target shooter with a solid understanding in basic ballistics.

I has a fantastic range trip this past weekend, and I observed some things I thought I share. I don't shoot a lot of M855, but decided to get a batch and try it out.

All I had with me was an AR with a 1:9 twist rate, I usually get very acceptable results with the 55gr XM193 with a respectable / consistent group pattern for what this ammo is capable of doing.

I started shooting XM855, while the general POI remained the same, the group pattern wasn't as clean or consistent as the XM193.

The only thing I could think of at the time is the 1:9 is having a little trouble stabilizing the heavier round. I was under the impression 1:9 was still within spec to stabilize the 62gr round.

The question: Is the 1:9 twist rate acceptable for the 62gr round, or should I really be using a 1:8 or 1:7 twist.

mif_slim
04-24-2012, 1:28 PM
Your particular barrel just doesnt like the 62gr. You can shoot other brands/grains and you'll yield different results due to load, speed, bullet design, harmonics, etc.

I have a 1:9 that can shoot all the way from 36gr-78gr out to 280 yards with no keyholes. You'll know when your barrel doesnt stabalize it enough. Keyholes is a sign of it.

MrPlink
04-24-2012, 1:28 PM
1:9 is fine for 855.

Ziggy91
04-24-2012, 1:36 PM
It's the barrel, they all usually have their own preferences.

For example, i get the opposite results as you. XM193 is probably the most inaccuarate and inconsistent 5.56 I've ever used, but M855 has been the opposite. I shoot about 1" groups at 100 through irons, and 1" groups at 200 with glass. I shoot through my 1:7 and 1:9 uppers. I also shoot 75 gr match out of my 1:9, which everyone also told me was a no no...

Every barrel is different, the only way to know is try or learn from others with the same equipment.

ExtremeX
04-24-2012, 1:40 PM
Your particular barrel just doesnt like the 62gr.



Ya, I was afraid of that...


You'll know when your barrel doesnt stabalize it enough. Keyholes is a sign of it.

I didn't notice any keyholes on my 100 yard targets, the grouping was bigger just wasnt as consistent as the 55gr rounds. Ill pay more attention to keyholes next time I get a chance to fire it, and will also try a longer distance.

ExtremeX
04-24-2012, 1:47 PM
It's the barrel, they all usually have their own preferences.

For example, i get the opposite results as you. XM193 is probably the most inaccuarate and inconsistent 5.56 I've ever used, but M855 has been the opposite. I shoot about 1" groups at 100 through irons, and 1" groups at 200 with glass. I shoot through my 1:7 and 1:9 uppers. I also shoot 75 gr match out of my 1:9, which everyone also told me was a no no...

Every barrel is different, the only way to know is try or learn from others with the same equipment.

Understood, this thread was really more to just confirm my findings... and share my experience. The 1:9 twist barrel I am using is the factory standard barrel from Stag Arms, which does great with 55gr XM193, but M855 round performance wasn't up to my expectations.

Unfortunately I left my 1:8 twist upper at home, so I never got a chance to really test it out with M855.

MrPlink
04-24-2012, 2:10 PM
how many groups did you fire before you came to this conclusion?

ExtremeX
04-24-2012, 2:25 PM
how many groups did you fire before you came to this conclusion?

Probably about 200 or so rounds of each ammo across the duration of the day... on fresh targets, from a bench at 100 meters, using an atlas bipod and monopod, with an ACOG in about 100 degree desert temperatures... I usually took breaks durring times of strong crosswinds so I would get more accurate results.

Agent Tikki
04-24-2012, 3:54 PM
M855 being steel core has accuracy issues, due to the inconsistency in the placement of the core. Its not unheard of for good rilfes to shoot 3 moa with M855. Unless you are keyholing, its not the rifling it's the ammo.

If you ever see anyone claim that they are consistently shooting 1 moa with M855, I'd call for BS.

Ziggy91
04-24-2012, 7:47 PM
If you ever see anyone claim that they are consistently shooting 1 moa with M855, I'd call for BS.

This was my average group from the last trip. I used M855 and shot this at 200 yards aiming at the bottom right corner through my 20" match upper which is "average" quality. According to you, I must have the holy grail of rifles if "good" ones shoot around 3 moa...

:D :D :D :D

http://i233.photobucket.com/albums/ee119/ZYGADLO2010/Snapbucket/AF59D372.jpg


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