Just a report on velocities for those interested.

Conditions: 40*F, 4000'MSL, cold guns, PACT Professional at 10ft

Ammo: Black Hills blue box remanufactured .223
75gr Match HP (non-moly)
68gr Match HP (non-moly)

Gun #1: AR-15 carbine
Barrel: 16", mid-length gas, stainless, Wylde chamber
75gr = 2465 fps Avg, 30 fps ES, 5-shots
68gr = 2590 fps Avg, 25 fps ES, 5-shots

Gun #2: AR-15 heavy rifle
Barrel: 24", DPMS .223 Chamber, stainless
75gr = 2634 fps Avg, 26 fps ES, 5-shots
68gr = 2811 fps Avg, 28 fps ES, 5-shots

Summary:
(Ignoring possible differences in chamber tolerances and thus relative pressure levels)

75gr load loses 169 fps in 8", or roughly 21 fps per inch.
68gr load loses 221 fps in 8", or roughly 28 fps per inch.

This was all very unscientific, and I just did it on the spur of the moment.  

Comments welcome to aid those who might use this info.

Great info!  Thanks for your time and effort!

good data, thanks for reporting Scotty. I didnt think velocities would increase that much with only eight inch difference...

Thanks guys.  The only way to tell for SURE how much difference there is would be to actually cut a barrel down so that the same action and chamber was used (I did this on my .308 about a year ago).

I imagine these results are pretty close to reality, though.  In a loose chamber, velocities would be slightly less, and in a tight chamber I would expect them to be a bit more....so actual MV will vary with individual rifles.

Really, this is typical for an efficient cartridge such as the .223.  ~25fps per inch is pretty good, as "overbore" chamberings such as Ultra Mags, 6mm/284, etc are more like 50fps/in.

Also, it really depends on WHICH inches we're talking about.  For instance, I would expect that inches 17 and 18 are really more like 30-35 fps/in, while inches 23 and 24 are closer to 18-20 fps/in.  There is a dimishing return involved, and it can vary with both bullet weight and powder type.

Actually, I expected the 75 to lose more than the 68, as in theory as faster powder for a lighter bullet would burn more efficiently in a shorter barrel.  However, I have no idea how BH is loading these powder-wise.  No doubt a good reloader could get the 75 moving a little better though choices of powders and tailoring the charge to the gun.  But....who (including me) wants to reload for a semi?

Now I've got to run the ballistics and see if the faster 68 will do better to 600 in my carbine......they're only $16/bx at my local Sportsman's Warehouse!

That's interesting as BH quotes 2750fps for a reference on the 75gr.

I'll go one step further and show a little hypothetical ballistics comparison for the 16" carbine.  I used the awesome JBM online calculator, which tends to be as good as any trajectory calculator out there.

The input was for my altitude of 5000', and I used the BC's of Hornady BTHP bullets, as I don't know exactly whose bullets BH is using for these loads.  For strictly comparison purposes, it should be fine.

The chart on the left is the 75gr, with the 68gr on the right, using the actual velocities from my rifle.

Now, as we can see, there is a definite advantage to using the 68gr at this velocity in terms of drop.  In terms of windage, the edge goes to the 75gr, but it is almost imperceptable until 700yds or so.  Also, the 75gr will stay supersonic slightly longer, provided somebody has the nutz to shoot a 16" AR to 900+.  

If someone was really looking at shooting past 600 with this setup, they are probably going to 1) reload their own; or 2) use more "premium" ammo.

So.....the final test will be which one shoots more accurately at, say, 300yds in the gun.  With unlimited time and ammo, it would be better, of course, to shoot both loads at a few 600yd matches and log the results and accuracy. When comparisons are this marginal, the paper doesn't always reflect reality.

Enjoy!

Quoted: That's interesting as BH quotes 2750fps for a reference on the 75gr.

What I'd like to see is about 20 guys here chrono the load in 20" guns, posting gun and environmental details.

That would give us a MUCH better idea of the cross-section of velocities to expect, as well as minimize hypothetical issues such as chronograph variation, etc.  

In this case, I had just shot through my tried-and-true .308 load minutes before, and it was right on the money as usual.


I wonder if the BH data is based on a 28" bolt-action varmint gun with a tight chamber....you know, reminicent of reloading manual data.

Quoted: Quoted: That's interesting as BH quotes 2750fps for a reference on the 75gr. What I'd like to see is about 20 guys here chrono the load in 20" guns, posting gun and environmental details. That would give us a MUCH better idea of the cross-section of velocities to expect, as well as minimize hypothetical issues such as chronograph variation, etc.   In this case, I had just shot through my tried-and-true .308 load minutes before, and it was right on the money as usual. I wonder if the BH data is based on a 28" bolt-action varmint gun with a tight chamber....you know, reminicent of reloading manual data.

Exactly.

The 68's and 75's in black hills are Hornady.  You done good.  

FWIW, I've heard tell that some of the Army Competition shooters, or someone like that uses black hills blue for competition.  I think they ARE premium loadings.

I actually called BH because I found the 75gr OTM was only clocking in at 2400fps out of a friends 1:7 twist 16" LMT upper. XM193 clocked in at 3100fps from the same barrel. The "tech" at BH indicated that they tested their .223 out of a 24" barrel and it averaged 2750fps. He indicated that he would expect about 2550 from a 16" barrel(approx 25fps drop per inch).  Interesting that you are also getting lower than expected velocities. I wonder what is going on here.

Quoted: I actually called BH because I found the 75gr OTM was only clocking in at 2400fps out of a friends 1:7 twist 16" LMT upper. XM193 clocked in at 3100fps from the same barrel. The "tech" at BH indicated that they tested their .223 out of a 24" barrel and it averaged 2750fps. He indicated that he would expect about 2550 from a 16" barrel(approx 25fps drop per inch).  Interesting that you are also getting lower than expected velocities. I wonder what is going on here.

What was the outside air temp at the time, out of curiosity?

I'll probably do a similar test later in the year when it gets hot to see if they're using a relatively temp-sensitive powder.  I'm betting they are.

Also, I have two other 16" guns to try (Colt barrels with 5.56 chambers).  Don't know when I'll get to it, though.

It was a cold morning for the desert. Roughly 45 degrees.