5.56 Ammo (M193 vs M855) grouping out of 1:7 twist barrel

[NJdiverTony 27]

So, I was at the range the other day with my AR... I have a 14.5" barrel with 1:7 twist... Theoretically, 62gr M855 should shoot better out of this barrel and give me tighter groups, but I found that regular 55gr M193 gave me much better and more consistent groups.

 

Can anyone tell me why this would happen? Just from what I've read, the opposite is supposed to be true (heavier bullets stabilize better and have tighter groups through a 1:7 barrel).

 

It's not a big deal... but it just made me curious. What do you guys think?

[Pizza Bob 1,488]

Because of the steel penetrator in the M855. If you had a plain FMJ 62 gr bullet you may find that it would shoot better than the M193. OTOH, all rifles have preferences as to what shoots best from them, and in your case it just might like the 55 gr stuff better.

 

Adios,

 

Pizza Bob

[raz-0 1,259]

855 with the penetrator core is going to be less accurate because of how the bullet is manufactured. The 55gr FMJ is a lead core jammed into a jacket. Most of the mass is at the center, and the worst case is you have some lack of concentricity at the edges due to voids or imperfect shape of the core and or jacket, which impacts balance. 855 has a steel cone up top, a lead core jammed into the bottom, and somehow get it all in a jacket around both. This effectively moves the majority of the mass away form the center, which magnifies the impact of any imbalance. You also have three components to make concentric and balanced, which is less likely to happen. Moving mass rearward thends to negate some of it. Generally, it is less accurate ammo, your results are not atypical.

[Shocker 151]

Shoot it at a different distance and see if the results are the same. If so your gun just likes the lighter bullets. If not you happened to be in an area where the instabilities did not present themselves the same.

[NJdiverTony 27]

See... I knew you guys would be able to able tell me! LOL. Thank you for the excellent and very informative answer!

With that being said... is there any other ammo besides XM193 that you guys would recommend for good accuracy at 100+ yrds out of my 1:7 twist, 14.5" barrel?

[checko 180]

http://ammo.ar15.com/ammo/project/perf_poi.html

 

That gives you an idea of the trajectory differences.

[Pizza Bob 1,488]

Hardly "plinking" stuff, but for max accuracy from factory ammo: Federal Gold Medal Match w/69 gr Sierra BTHP.

 

Try some Black Hills with the heavier bullets for a more cost effective solution - still not cheap.

 

Adios,

 

Pizza Bob

 

Federal AE makes a 62 gr FMJ .223 load that works well in my AR

[checko 180]

Do you re-zero for each?

[NJdiverTony 27]

Shoot it at a different distance and see if the results are the same. If so your gun just likes the lighter bullets. If not you happened to be in an area where the instabilities did not present themselves the same.

 

I tried shooting several seperate groups at various distances at the range, to see the results. The M855 consistently gave me groups that were 2 to 3 times larger than M193. At 50 yrds, I had .5" groups with M193, whereas the M855 gave me 2" groups. At 100 yrds, I had 2" to 3" groups with M193 and 5" to 8" groups with M855. It was a really big difference.

[NJdiverTony 27]

Do you re-zero for each?

I know that their trajectories are different... so I was shooting to observe groupings, not for bullseye's... so there wasn't a need to re-zero. As long as I was aiming at the same spot to ensure consistency, then that was fine.

[raz-0 1,259]

See... I knew you guys would be able to able tell me! LOL. Thank you for the excellent and very informative answer!

With that being said... is there any other ammo besides XM193 that you guys would recommend for good accuracy at 100+ yrds out of my 1:7 twist, 14.5" barrel?

 

Different guns will like different things. One of my guns absolutely LOVES the cheap remington 55gr JSP as well as wolf 62gr (while they were using actual copper jacketed projectiles). None of my other guns shoot either nearly as accurately.

 

Technically, a flat based jacketed soft point should shoot better than a same weight FMJ with similar QC applied. IT should have a higher BC and have a reward shift in weight to help. Doesn't mean your gun will like it.

 

If you are looking for something affordable, I'd try hornady steel match. It's not cheap cheap, but it is within a reasonable range compared to m193/m855 brass cased clones and has decent bullets in it. The 55gr jhp rounds aren't too pricey, and the 75gr bthp rounds are about the cheapest you can find a good bullet loaded in, but still aren't bulk plinking ammo prices.

[NJdiverTony 27]

Different guns will like different things. One of my guns absolutely LOVES the cheap remington 55gr JSP as well as wolf 62gr (while they were using actual copper jacketed projectiles). None of my other guns shoot either nearly as accurately.

 

Technically, a flat based jacketed soft point should shoot better than a same weight FMJ with similar QC applied. IT should have a higher BC and have a reward shift in weight to help. Doesn't mean your gun will like it.

 

If you are looking for something affordable, I'd try hornady steel match. It's not cheap cheap, but it is within a reasonable range compared to m193/m855 brass cased clones and has decent bullets in it. The 55gr jhp rounds aren't too pricey, and the 75gr bthp rounds are about the cheapest you can find a good bullet loaded in, but still aren't bulk plinking ammo prices.

 

Thanks for the info. Much appreciated.

 

The best would probably be for me to start reloading .223 and come up with a recipe that my AR really likes. I already reload for pistol but have been putting off reloading for rifle, as it's kind of a pain in the ass and time consuming (as compared to pistol ammo loading).

[mipafox 438]

First you said "M855" and "M193" and in a later post you said "XM193."

 

M855 is normally somewhat more accurate than M193 out of most 1/7 barrels. It's been proven zillions of times. Slower twist, other things may come into play even if the M855 is still stabilized.

 

If you are using XM855 and XM193, not M855 and M193, then the answer is product variability in these commercial products. And a variability that has historically been worse when it comes to XM855 from Lake City operators. I haven't shot any XM855 from Lake City lately, but the reputation was very bad for a very long time. And now that the stuff at least goes bang when you pull the trigger, people testing it find a high variability of accuracy.

 

If you want to shoot M855 from civilian sources, it seems like the Israeli stuff is pretty consistent. Perhaps Prvi after that.

[NJKen 23]

Thanks for the info. Much appreciated.

 

The best would probably be for me to start reloading .223 and come up with a recipe that my AR really likes. I already reload for pistol but have been putting off reloading for rifle, as it's kind of a pain in the ass and time consuming (as compared to pistol ammo loading).

That would most certainly be the best route to accuracy. I just acquired a FAR15 with a 20" 1 in 8 barrel and I got a great group out of 69gr sierra match kings. I dont know if you can buy any commercially loaded ammo with these bullets.

I also shot some XM193 and XM855 and I got about the same results out of both. The difference was too close to call one better than the other. The 69gr was clearly superior.

Ken

[MandM 2]

Beyond twist rates... shouldn't barrel length play a slight role as well?

 

If not, then why do scope manufacturers that have caliber specific bdc reticles tell you what bullet, gun, barrel they calibrated with? It's gotta be more than just bullet weight or else it would be inconsequential for them to tell you "62 gr. 5.56 NATO, 20" barrel, etc." right?

[raz-0 1,259]

Beyond twist rates... shouldn't barrel length play a slight role as well?

 

If not, then why do scope manufacturers that have caliber specific bdc reticles tell you what bullet, gun, barrel they calibrated with? It's gotta be more than just bullet weight or else it would be inconsequential for them to tell you "62 gr. 5.56 NATO, 20" barrel, etc." right?

 

Barrel length affects velocity. Velocity affects the time of flight, which affects the bullet drop at a given range. The ballistic coefficient of the bullet matters too. It really doesn't have much to do with accuracy beyond barrel harmonics. For that, there will be an optimum length, and longer or shorter is worse. It'll vary with variations in bullet, so.... you can chase it, but you use barrel weights for that, not altering barrel length.

[JimmyAGR 54]

That would most certainly be the best route to accuracy. I just acquired a FAR15 with a 20" 1 in 8 barrel and I got a great group out of 69gr sierra match kings. I dont know if you can buy any commercially loaded ammo with these bullets.

I also shot some XM193 and XM855 and I got about the same results out of both. The difference was too close to call one better than the other. The 69gr was clearly superior.

Ken

 

You can get SMK 69's in both Federal gold medal match and Black Hills. Both are very nice loads. They are around $1.00 per bullet.

[NJKen 23]

You can get SMK 69's in both Federal gold medal match and Black Hills. Both are very nice loads. They are around $1.00 per bullet.

Good to know. I can load them myself for less than that but in a pinch its nice to have options.

Ken

[raz-0 1,259]

Barrel length affects velocity. Velocity affects the time of flight, which affects the bullet drop at a given range. The ballistic coefficient of the bullet matters too. It really doesn't have much to do with accuracy beyond barrel harmonics. For that, there will be an optimum length, and longer or shorter is worse. It'll vary with variations in bullet, so.... you can chase it, but you use barrel weights for that, not altering barrel length.

 

Just realized I left something out. shorter time in flight gives you less time to be affected by wind. It doesn't make it more accurate, but it does make it a bit easier to hit what you want to hit. HOWEVER, to go faster usually means a lighter bullet, which means it is more sensitive to wind even though it is exposed to less of it. A longer barrel gets you some speed without changing bullet weight.

[MandM 2]

Makes sense! Thanks for the info!