Rifle Round Transition to Subsonic

[MedievalEngineer]

I got the chance to shoot at a 600 yard range recently and brought what I had which was a 5.56 16" AR with a 1:9 twist and XM193 55 grain ammo. This combination worked okay out to 500 yards but 600 was a bit of a disaster, particularly when the wind picked up. A new upper is in my future but not soon enough so I'm stuck with the barrel for now. My current plan is to reload with 69 grain Sierra HPBT bullets before my next outing. I loaded up some test rounds and the results surprised me a bit. I loaded 5 rounds each with 23.5gr, 24gr, 24.5gr and 25gr of W748 with CCI #41 primers, all with a light chrimp on full length resized and trimmed LC brass. There was a clear trend with the 23.5gr having the best group of 5/8" at 100 yards, the 24gr around 7/8" and the 24.5 and 25gr both being around 1.125".

No visible pressure signs on any of the rounds and Sierra load data says I should be able to go up to 25.8gr. I'm conflicted on whether I should stick with the low velocity load or try to find a high velocity load that works. My concern is that my muzzle velocity is probably only around 2500 fps and based on my ballistic calculator, my velocity at 600 yards will only be around 1025. How bad is this? I'm only shooting paper (or perhaps some steel) but it seems like dropping in into the subsonic range will kill accuracy. Any advice would be appreciated.

 

[BradMacc82]

2580fps range for 23.5gr is where your likely at for that 16".

Going by that, your going to be at roughly 1204fps at 600 yards, still safely above the subsonic transition.

ETA: Lets just say for sake of argument your only clocking in the high 2400fps range - say 2470fps, that still puts your downrange velocity at roughly 1150fps.

 

[stu929]

Not to jack what program are you using? I need one.

Also some 1/9 like 69s some dont. Check out white oak great uppers.

 

[BradMacc82]

http://handloads.com/calc/ is what I've been using.

 

[BradMacc82]

Now if you really want an education - Paging E.Shell, paging E.Shell, your vast wealth of knowledge is needed.

 

[stu929]

http://handloads.com/calc/ is what I've been using.

Thanks. I dont have a chrono yet unfortunately and the load books vary so much on velocity.....

 

[BradMacc82]

Thanks. I dont have a chrono yet unfortunately and the load books vary so much on velocity.....

It's not a bad program, it puts you safely in the ballpark - at least for me it has with the loads I've been able to run out to 300 yards.

 

[stu929]

I just mean I have no way to establish MV. I've been using the strelok app on my phone.

 

[IndispensableDestiny]

2580fps range for 23.5gr is where your likely at for that 16".

Going by that, your going to be at roughly 1204fps at 600 yards, still safely above the subsonic transition.

ETA: Lets just say for sake of argument your only clocking in the high 2400fps range - say 2470fps, that still puts your downrange velocity at roughly 1150fps.

The bullet is still supersonic, but likely in the transonic regime which begins at about 1340 fps.

The OP should look for an accuracy node that's closer to the max. It's there, somewhere.

 

[MedievalEngineer]

That's good news, I rechecked my numbers and I think I found the problem. I used the same ballistic coefficient for the 69 grain bullets as I was using for my 55 grain bullets. I was using .244 when it looks like I should have been using around .310 I think on the next trip to the range, I'm going to prep another group with that same load and bring the chrony to see what my actual muzzle velocity is. I thought about bringing it today but I only had 5 rounds of each and didn't want to try to shoot for groups and get velocities at the same time.

 

[Speed3]

Now if you really want an education - Paging E.Shell, paging E.Shell, your vast wealth of knowledge is needed.

Agreed I feel like I learn something every single time he posts

 

[BradMacc82]

Agreed I feel like I learn something every single time he posts

We've been exchanging PM's the past few days, there's an incredible amount of knowledge stuffed into that man's head.

 

[Pinecone]

A great way to develop loads - http://optimalchargeweight.embarqspace.com/

Based on OCW, there is an accuracy node every 3% change in charge weight, with a scatter node inbetween each accuracy load. To capture this, you should shoot a test with the charge weight changing between 7. - 1% per step. 0.5 grains in your range is 2% per step, so you can skip over accuracy nodes.

And shoot them in a round robin. If you ahve 5 charge weights (A - E) and 3 rounds each (A1 - A3). Shoot A1 at target 1, B1 at target 2, C1 at target 3, to E1. Then A2 at target 1, B2 at target 2, etc.

This averages out barrel fouling and heating affects.

And wait 2 minutes between each shot (bolt open) for cooling.

But a 69 grain should be good to 600 yards. This was the first heavier bullet designed to make the AR shoot well at High Power Matches. The 77 grain Sierra Match King would be even better.

As for ballistic calculators, I use JBM online - http://www.jbmballistics.com/

On my iPhone, I use Ballistic AE, which uses the JBM "engine."

 

[Biggfoot44]

With chronographs starting at under $100, they are an important tool for any semi-serious handloading.