Calculating MV from Shorter Barrel rifle

[bwsmith]

I put together an Encore 308 with a 16.25" barrel and am setting it up for hunting. While I dial in a hand load I am using soem fatory ammo in Barnes VorTX 168grain with a printed muzzle velocity of 2680fps. I believe that this number comes from a 24" barrel. I am trying to figure out an estimated MV from my short barrel to figure out holdovers and where I should sight it in at. Any help is appreciated.

Bullet details.
DIA. .308
WGT. 168 GR
LGTH. 1.418
S.D. .253
B.C. .470

 

[adelbridge]

figure you will loose about 20-40 fps per inch. If you have access to a chronograph you can calculate velocity with bullet weight to get true numbers. I am sure you can get data off the web for similar barrel length and load for a close approximation.

 

[dubbleA]

Each and every barrel is different, some are faster than other while some are slower. It's not an exact science. Some factory claims on velocity are very optomistic and are done in contolled conditions.

Anything other than actually chronographing a particular load is just a guess.

Once you get the velocity then you can figure that elavation and temperatures will play a part. While ballistic charts are available most anywhere here on the net I only consider them guidelines and will always suggest actually going out and shooting the distances you plan at shooting at. You can then get true figures for your rifle and your load.


In my experinces with working with a few 308's of mine, cutting them from 26" to 20" I lost and average of 75fps doing so. The diminutive 308 Win is a fairly efficient cartridge and doesnt seem to loose much velocity compared to bigger cartridges using more powder.

 

[jmr40]

I've found that 15-20 fps/inch is pretty close with barrels 20"-24". Once you drop below 20" the velocity starts to drop in bigger chunks, closer to the 40 fps quoted above. There are so many factors to consider that it is hard to predict with any accuracy. Individual barrels will shoot to very different velocities. I wouldn't be surprised to see some 24" barrels shoot right at the published velocity while another might be 100 fps slower. My best guess from a barrel that short is somewhere between 200-300 fps slower than advertised.

I like the Barnes bullets, but don't think the 168 gr bullet is a good hunting bullet for this application. Barnes bullets need SPEED to work properly. Drop below 2000 fps at impact and they don't work well at all, 2200 fps is prefered. The 165 and 168 really need magnum speeds to get reliable expansion. From a barrel that short I'd be shooting 130 TTSX's or go with a Berger VLD or Hornady SST in a heavier bullet.

I'm loading 130's at 3050 fps from a 22" 308. They penetrate and expand equal to a conventional 165 gr bullet.

 

[ohwell]

You would have to use a chrono to know for sure. There are other variables that would would also have an effect such as powder burn rate.

 

[bwsmith]

I like the Barnes bullets, but don't think the 168 gr bullet is a good hunting bullet for this application. Barnes bullets need SPEED to work properly. Drop below 2000 fps at impact and they don't work well at all, 2200 fps is prefered. The 165 and 168 really need magnum speeds to get reliable expansion. From a barrel that short I'd be shooting 130 TTSX's or go with a Berger VLD or Hornady SST in a heavier bullet.

I started at 125 and 150 grains but couldn't get anything to group. I got the 168gr Hornady Match to group MOA. I guess that it needs that heavier bullet to stabilize or burn up all the powder in the short barrel. I am working up loads trying 165 SST and 168 Nosler Ballistic Tips. These are to get me through for a couple weekends until I can get back to the range to test some loads. Just trying to get a decent estimate on the factory stuff out to 200 yards.

 

[dubbleA]

I like the Barnes bullets, but don't think the 168 gr bullet is a good hunting bullet for this application. Barnes bullets need SPEED to work properly. Drop below 2000 fps at impact and they don't work well at all, 2200 fps is prefered. The 165 and 168 really need magnum speeds to get reliable expansion.

Sometimes speed doesnt help and things dont work as advertised.

This 168gr TSX took a fine Canadian whitetail at 104 yards with a head on slightly quartering to me. Shot dead center between the front shoulders be bullet was found in the rear left ham. Penetration was good but exspansion left a little to be desired. Fired from a 300WSM with a chrono'd average velocity of 3080. Yes, the shot was fatal as the buck only went a few yards with zero blood.

 

[lefteyedom]

you have t buy a chrony, I thought only Lawyers and car salesmen could tell such ball face lies. But factory ammo and reloading manuals take the cake.

As said before every barrel is different. the 1 inch from 24" to 23" is a much smaller percentage of the whole than the 1" from 17" t 16"


have fun