Norma Match going supersonic?

[Hookturnr]

Anyone else had this happen to them? I was running through some practice today and had 10-15 rounds go supersonic and crap my accuracy..

I'm shooting Norma Match in a CZ 455 17 1/2" barrel with a Surefire Ryder ss. Still the factory varmint barrel just shortened and threaded for the can.
Weather was as follows: 63 deg. 83% humidity 1200'ASL bar 30.2 falling varying tailwind of 5-15mph. on/off rain

I have been shooting this stuff for a couple years now and never had this happen until today. Any ideas?

 

[TimeWillTell]

Sorry I'm confused...
What caliber? What velocity do you normally get and what is the "supersonic" velocities you're getting? (You said supersonic a couple times so I'm guessing these are supposed to be sub loads?)

 

[Hookturnr]

22lr. I normally get 1075fps with this ammo and stay well below supersonic but there is no doubt when you get a supersonic crack while running a suppressed 22lr
I wasn't shooting over a chronograph today but the rounds that had an audible crack also hit well off target, typically high

 

[Hookturnr]

Just to fill in everything I can think of about the day..I was shooting 50yds, 100yds, and 200yds intermittently and from various shooting positions.

This is my typical practice routine and I have never had this particular thing happen before...thus the reason i ask if anyone else has had this particular ammo go supersonic. It's irrelevant at short range but when you get out to 200-300 yds it absolutely destroys accuracy

 

[Snuby642]

So i figure a 6-7 % deviation could get you cracking?
Evidently colder weather can help make that happen.
I shot 1070 fps 18" barrel and got mixed results on cold days as to the crack.
Same time shot 1050 and it did not , also the 1050 was more accurate that day.
Also pouring rain off and on with fog, could only see 50 yd well.
Going on posted fps from mfg.
So maybe 2-3% for weather and 2-3% of a deviation in ammo?
Just checked the norma clocks in at 1100 fps.
I think that would crack well before a lot of other 22lr.



I don't know if my 6 inch can increases velocity or not.
Got this off Wikipedia for a reference.

Subsonic rounds have a muzzle velocity of about 1,080 ft/s (330 m/s) or less,.Accuracy is reportedly improved with subsonic rounds because a supersonic bullet (or projectile) that slows from supersonic to subsonic speed undergoes drastic aerodynamic changes in this transonic zone that might adversely affect the stability and accuracy of the bullet.
Because the speed of sound in air at 68 °F (20 °C) is about 1,115 ft/s (340 m/s), the subsonic round's muzzle velocity is slightly below the speed of sound under many hunting conditions. However, under cold air conditions at 32 °F (0 °C), the speed of sound drops to 1,088 ft/s (332 m/s), approximately muzzle velocity. Hence, a "subsonic" round used below this temperature may be supersonic, and during the transition from supersonic to subsonic velocity, it may become unstable, reducing accuracy. To counteract this, some cartridge manufacturers have lowered the speed of their subsonic ammunition to 1,030 ft/s

 

[Hookturnr]

I've chrono'd a bunch of this ammo and come up with a solid average of 1075fps. I'm thinking it may have just been the right combination of temp/humidity etc. to just get over the sound barrier. I just have never had this ammo go super on me before this. There's not a lot of deviation required when you're this close to the sound barrier, apparently I found my way into that zone...

 

[Snuby642]

I think so as well, per mfg fps, (no crono) 20 listed fps put us in the crack zone.
I think we had near 40 for a high that day, after the rangemaster tossed us out because we could not see the range was clear anymore
We tested a handfull of ammo in our indoor/outdoor facility and nearly everything in the 1070 range cracked, but suppressed.
Do you have any data from your crono on suppressed vrs unsupressed since we have similar barrels?
What altitude were you at, it also goes into the mix for sos but I'm not a mathematics wiz we were at about 150 ft.

 

[natdscott]

1) It's SLIGHTLY cool. Wax that is otherwise fairly fluid can harden up under colder temperatures, leading to higher pressure...thus, velocity. This is the reason why companies like Lapua make biathlon-specific loads, with special powders, alloys, and waxes to keep them under super. Given 63 degrees though, I don't consider this much influence.

2) Your barrel is too short to start with. This is the reason why match rifles have very long tubes relative to what the "average" shooter thinks of for a "Twenty Two". The whole damn trend towards "18" .22 does everything" is based on either false logic, or on using supersonic ammo only. Super is FINE. Sub is FINE. It's the transition that f(*^s you.

3) The suppressor can be adding speed, no doubt. I have to run 3 ammunition types in my rifle. 1) match loads for the summer, (2) field loads for the winter that STAY under, (3) general purpose summer loads for snap shooting, field shooting, etc. I also keep heavy low-speed subs around for the occasional thing that needs a hole with NO sound. Yes, I have to have dope for all three main loads. Gosh, ain't life grand when you're spoiled with hundreds of ammunition types?

-Nate

 

[Snuby642]

Thank you, I keep forgetting the wax factor as I have just recently went to match ammo for rimfire.
I run a sparrow can, 5 inch any estamate of the velocity increase?

 

[Hookturnr]

I see about 20fps increase with my 6" surefire.
Im at 1200 ASL

As for super vs. Subs, my chosen game (rimfire PRC) has me shooting anywhere from 30 yds to 300yds so it dictates subsonic only to keep to one dope sheet and zero for a given match.

Ive got a couple long barrel guns for other 22 games and they definitely perform well, they just dont fit this type of comp. as well as the shorty cz

 

[Hookturnr]

I hadnt considered the lube factor either, but given the temp, it was still fluid that particular day.

I put 150 rounds through it yesterday with lower temp, lower humidity and lower pressure and didnt have any crack. I put 25 of those over a chronograph as well and ranged from 1059 to 1107 for the two extreme readings and all the rest within 8fps of 1075.

That is well within the error of the chrony so I'm going to call it good.

 

[natdscott]

You know what a good solution is as well?

Shoot 1050 ammo in short barrels, in the cold, and (apparently) with cans.

 

[Snuby642]

Was with an ammo manufacturer today putting in an order.
Mentioned the breaking sonic to them.
Was told that the high humidity and cold/ cool temps have a strong effect on subs.
They make a lot of subs and that was why they kept thiers slower than others.
They didn't want to teter on the thin line on purpose evedently
I had wondered in the past about the missing exra 50 fps, now I know it was intentionally done.
I'm guessing that the lower humidity kept you under the line.

I will try not to scream like a little kid next time I have it happen.

 

[Hookturnr]

I guess I'll have to look into switching to a different match round in the future.
Right now I've got a couple bricks of this case left and a few matches coming up in warmer weather so...same old same old for now

 

[Snuby642]

Keep the norma and get some super crap for a (winter round) for the ocasional crap conditions.
I'm going to do some super on a different range card for weather and maybe long ranges.
That way I dont worry about the surprise transition, may be better than the good stuff on a bad day.
In fact I'm going to get something inexpensive on dope on purpose!
Many guys on the forum have said winter/summer rounds.

In Texas we have 2 seasons, target and critter. Only half of critter season is cold enough to notice.
Never did much target shooting during the cold in the past just minute of critter accuracy was good.
Still learning a lot on the hide!

 

[Hookturnr]

I've got a different critter round already, CCI subsonic segmented HP, that shoots great in typical tree rat hunting temps.

I've run through a bunch of different brands/lots with this gun and still have a couple I was planning on testing anyways for match use.
If i find myself hunting critters of another flavor or longer ranges, I've got a 22mag that shoots sub MOA to 150yds.