Why would 6.5cm and 308 have less impact on velocity with shorter barrel than 270?

[steffen707]

so i've been reading a lot about going from 24" to 16-18" and most folks say that the larger caliber of 308 has less fps drop than say a 270, but then I just watched this video, and it looks like the 6.5cm barrel they tested cutting down by 1" at a time faired very well with only 25fps loss per inch cut.?

 

[TxxAgg]

Those findings match up with the two T3x 6.5s I've owned. One cut from 24" to 21" and one cut from 24" to 18".

Just a note...Both started off about 100 fps less than the box advertised.

 

[AZ_Hunter_2000]

Here are a few examples of real-world gains and losses:

I have a 20" 308 Win (Savage Lightweight Hunter) that I run Hornady Precision Hunter through. The box states 2600 fps with a 24" barrel; my actual velocity is 2518 fps. So on this rifle I lost 20.5 fps per inch.

I have a 22" 6.5 CM (Savage Storm 110) that I run both Hornady Precision Hunter and Hornady Match through. For Precision Hunter, it states 2700 fps with a 24" barrel and I actually get 2726 fps. I gained 13 fps per inch "lost" with the Precision Hunter ammo. With Match ammo, it states 2695 fps with a 24" barrel and I actually get 2685 fps. I only lost 5 fps per inch.

I had a 22" 6.5 CM (Savage Axis II) that I ran Hornady Precision Hunter through. The box states 2700 fps with a 24" barrel and I was getting 2681 fps. So on this rifle I lost 8.5 fps per inch.

When it is all said and done, your rifle will dictate the actual velocity loss/gain. The 25 fps per inch is just a guideline.

 

[hodgeman]

The powder capacity to bore ratio dictates the loss. The actual barrel also factors into the equation...some barrels are just "faster" than others. I've seen some pretty dramatic losses with rounds like the 270 and 300WM when barrels are chopped. I've seen much less loss on rounds like the .308 and 7-08, so little in fact that I think a 22-24" barrel on either is just a waste of metal...18-20" is very much right there. My 22" 6.5 is faster than my 18" 6.5... but it's a whopping 50fps with most "book" loads. With slow burning powder, that spread widens considerably.

The ballistics printed on the box are relatively worthless when considering any individual rifle.

 

[SDHNTR]

Short vs long case, less vs more powder to burn. A 308 is going to burn up mostly all of the powder in its case as it travels the length of an 18 or 20 inch barrel. A 270, for example, has more powder to burn, and thus, will need a few extra inches of barrel to burn it all.

 

[Formidilosus]

As I’ve written before, most of the “insert efficient short barrel cartridge” nonsense, is a myth. When using optimum loads for each, expect 15-30fps per inch. I have shot plenty of short barrel 223’s, 6.5’s, 308’s, 300 and 338 mags. Some will lose more, some less, but when optimized for their length- 20-25fps is what I would expect per inch.

 

[steffen707]

My bad, I should have said I heard you lose like 25fps per inch. I've seen that on the 308, but then I was surprised when it was only 25fps on the video I posted with the smaller than 270 caliber 6.5cm. Because people kept saying with smaller caliber you'll lose more per inch.....this was about the same.

This makes me think @Formidilosus may be right.

I've seen 223, 6.5cm,308 and 300wm. Need to see more inch by inch testing. Maybe I'll be the fool to try it with a 270wsm before I rebarrel it for 6.8 Western.

 

[steffen707]

One thing I don't like, is the 300win mag result was only with 1 round used instead of an average of 4. The 7mm rem mag is an oddball, but i think its because it was cut from 26-20", may have lost a lot more from 20-16", but the tester said it was VERY LOUD.

Most links are from rifleshooter.com go there and search for the results.

 

[steffen707]

I searched case capacity / bore and found this chart

.224" - 22-250Rem------- 43.5gr-- 194
.243" - 243 Winchester--- 54gr---- 222
.257" - 257 Roberts------- 56gr---- 217
.264" - 6.5x55 Swede----- 57gr---- 216
.277" - 270 Winchester--- 67gr----- 241
.284" - 280 Remington---- 67.5gr-- 238
.308" - 30-'06 Springfield- 68gr---- 220
.308" - 300 Win Mag------ 90gr----- 292
.323" - 325 WSM---------- 82gr----- 253
.338" - 338 Win Mag------ 86gr----- 254

.264" - 26 Nosler----------93gr------352
.308" - 300 RUM-----------113gr----366
.323" - 8mm Rem Mag----98gr-----303
.375" - 375 Ruger---------99gr------264
.510" - 50 BMG------------292gr----572

Case capacity to bore size ratio

www.shootersforum.com

 

[sndmn11]

Here is our numbers with a magneto speed, take them with a grain of salt.

Weatherby Vanguard Camilla 7-08 20" plus accubrake
-2820 Hornady Superformance 139gmx (factory 2910)
-2869 Hornady Superformance 139sst (factory 2950)
-2891 Barnes Vortx 120ttsx (factory 3005)

Tikka T3 with 7-08 Preferred Barrel Blank 20"
-2890 Hornady Superformance 139gmx (factory 2910)
-2891 Hornady Superformance 139sst (factory 2950)

Weatherby Vanguard Compact 6.5cm 20" plus Hawkins brake
-2674 Hornady American Whitetail 139interlock (factory 2820)
-2752 Federal Premium 120trophy copper (factory 2875)

 

[hodgeman]

The case capacity to bore ratio is as described above. Bottom line...the more powder you try to burn in a tube of a certain size...the longer that tube has to be to burn it all. You can change powder characteristics and alleviate that to some degree if you optimize your load.

Some cartridges have a bunch of powder and a relatively small bore- like the .264WM or anything Weatherby. You start chopping inches off the barrel and the speed drops quite a lot. Very few people have carbine .300WBYs...because there's little point to making the world's loudest 30-06.

Other rounds have a very good capacity to bore ratio- like the .308 or 7-08 that burns almost completely in a 20" barrel.

You can also look at it from the other direction, once a .22LR barrel gets over 16"-17"...the MV just doesn't increase. My 24" barreled CZ isn't a lick faster (and may even be a little slower) than my carbine version. There's no more powder to burn and peak pressure happens before the bullet leaves the barrel. Pistol caliber rifles are a good example as well.

 

[steffen707]

ah crap same site has info on a 338 lapua, 6creedmoore 224 valkyrie, 7.62x39 and 243
i'll have to revamp my chart........

I updated the chart from post 8.​