Transonic bullets... what is actually going on here??

[turnburglar]

Hey Shooters,

I wanted to get the forums opinion on long range shooting; specifically with the Ar15 and 556 platform. First off let me introduce the rifle:

14.5" P&W Faxon match gunner 1:8 wearing a Steiner P4xi; that chronos at 2850 FPS with American Eagle 55gr.

This is my competition rifle that typically gets used at 0-400 yards. Because my local matches are range limited to 400, I have never felt terribly handicapped with the shorter barrel, but when the winds start picking up there has been times when I wish I had a faster bullet to buck the wind a little. I recently signed up for a (new to me) match called "accurized AR" where they claim to lay out each stage with either more precision oriented targets if under 250, and over 250 they say the targets are generous but can be as far out as 700 yards. I mentioned this to my shooting buddy and he said that my rifle might struggle going transonic at the longer targets. I punched the number into Strelok and found my rifle goes transonic at 600 yards with 55gr and 700 yards with 77gr loads. My buddy suggested I shoot my 77's but I dont like how much those actually drop. They are 223 pressure reloads and only making like 2450 outa my barrel. Currently the rifle is zero'd at 50 yards with 55gr Horandy M193. Thats the lot of ammo I was thinking about taking to this match, but wanted to hear the boards feedback.

Im even willing to chalk this up as a learning experience if my 55gr loads isn't ideal, but 'good enough'. Because of the market I can't easily pick up new ammo before the match and have to use what's on hand. What should I expect?





The target in the reticle is at 600 yards with 0 wind value. If I add 2 MOA to my zero it will make that bottom hash mark 550 yards.

[sinister]

What should I expect?

If it's windy and you don't stay up with the changes you'll lose.

No big deal, the targets aren't shooting back, right?

You might try Sierra 69s, tipped 60s or 69s, Hornady 68s, or Berger 73s -- but they're all going to have more drop than 55s. You can't fight (the laws of) physics and gravity. A lighter bullet will be faster, initially, but sheds velocity and the ability to buck wind the farther it travels.

[turnburglar]

Yea, I dont compete to win; mainly just to grow as a shooter. The match fee is only $25, so needless to say I'll be out more in gas and ammo.

But will the 55gr bullet still be flying point first when it hits transonic speeds? Are the holds generated by strelok still accurate, or is the bullet just a wild card at that distance? I have no experience pushing these to the limit.

[lonestardiver]

There is no telling what the bullet is going to do as it transitions from supersonic to subsonic. In the .308 world the 168 gr goes transonic around 800 yds is unstable. If your 55 gr goes sideways you introduce a lot more drag and you might lose too much velocity not to mention it will drop faster then.

[Bimmer]

But will the 55gr bullet still be flying point first when it hits transonic speeds?

Play with bullet stabilization calculator:

https://bergerbullets.com/twist-rate-calculator/

As far as I can see, even down to 1,000fps, a 55gr. .224" bullet is VERY stable.

There is no telling what the bullet is going to do as it transitions from supersonic to subsonic.

Right, and there are all kinds of weird things happening with drag coefficients around the sound barrier, too.

I think, if you want to shoot out to 700yds, you need to figure out someway to stay comfortably supersonic.

[Vegas]

I think, if you want to shoot out to 700yds, you need to figure out someway to stay comfortably supersonic.

77gr SMK at 2750 will get it done in my locale. My calculator shows it is still at 1385 at 750 yards. This is from an 18" barrel. With a 16" barrel I show 2617 and 1330 at 750 yards.

[turnburglar]

Play with bullet stabilization calculator:

https://bergerbullets.com/twist-rate-calculator/

As far as I can see, even down to 1,000fps, a 55gr. .224" bullet is VERY stable.




Right, and there are all kinds of weird things happening with drag coefficients around the sound barrier, too.

I think, if you want to shoot out to 700yds, you need to figure out someway to stay comfortably supersonic.

Hey thanks for the reply, I put in similar conditions for where I shoot and even down too 550 FPS it appears the 55grs are still getting an SG of 2+. This puts my mind at ease knowing they should still be flying and not turning sideways.

I want to take my 6.5 Creedmoor to this match, but there is a slim chance I will be able to get enough ammo in the current climate.

[Bimmer]

See here, and scroll down to figure #7:

http://article.sciencepublishinggrou...150306.14.html


This is what I was talking about in terms of drag around the sound barrier... Slowing down to Mach 1.3 or even 1.4 means the drag goes waaaay up, which slows your bullet further, which increases the drag more: viscious cycle!


Note, too, that this is why the most accurate ammo for .22lr is always subsonic: it stays away from the bad "transsonic" speed by staying well below Mach 1.

[markm]

If bullet drop is your concern. Try Black Hills 69 gr TMKs in 5.56 pressure (if your chamber will have it safely)

Nothing that I know of in 556 comes close to how flat this round shoots.

[Joker]

Have you tried the IMI razor core 77gr loads?