Optimal barrel length for 308

Detailled example using 175 SMK

I did detailled calculations using 175gr SMK at 2500fps in 16"
barrel and 2750fps in 24" barrel (experimental data).
The short answer is that you get the equivalent of 1% improvement in BC of bullet for each 1" of barrel from 20" up to 27" inches.
I would not extrapolate beyond these values since my velocity curved stopped there!

Barrel length Velocity 2680fps equivalent BC Drift 1000yards 10mph
---------------------------------------------------------------------
16" 2500fps 0.445 G1 (6% worse) 114"
20" 2620fps 0.465 G1 (2% worse) 108"
22" 2680fps 0.475 G1 (baseline) 105"
24" 2740fps 0.485 G1 (2% better) 102"
27" 2830fps 0.500 G1 (5% better) 97"

If all things are equal, a longer barrel gives higher velocity as long as the force from expanding gas (pressure * bullet surface) is greater than bullet-barrel friction.
Friction and pressure is pretty much constant near the muzzle since total barrel-chamber increases linearly and that friction is almost constant.

Again, not a quantum leap in performance but a 8" decrease in wind drift when increasing barrel length from 22" to 27" inches.

Alex
 
So, lets get this straight: you assigned arbitrary/theoretical velocity values for each barrel length, then ran them though a ballistics calculator... And this proves what exactly?

There are guys on here who actually have 20" barrels (and load for them) that will laugh at those figure. I will for the 22" velocity listed since that's what I'm currently running on my 308. And incidentally, I can run a 208 AMAX out of that barrel at 2560 fps. with RL-17 and it will only have 72" of wind drift. Bullet and powder choice have a larger effect than barrel length, especially with the newer ones being offered. The longer barrel will only have an advantage if you optimize these for it or you add enough length to hit a higher node and your powder hasn't already attained a complete burn in the shorter barrel. The thing is most guys don't; they shoot a bullet and powder combination that makes their few inches of barrel length meaningless because it doesn't allow them to hit higher nodes than the guy with the shorter barrel. The guy with the shorter barrel can outperform them by simply being smarter about his powder and bullet choice.
 
kombayotch said:
So, lets get this straight: you assigned arbitrary/theoretical velocity values for each barrel length, then ran them though a ballistics calculator... And this proves what exactly?

There are guys on here who actually have 20" barrels (and load for them) that will laugh at those figure. I will for the 22" velocity listed since that's what I'm currently running on my 308. And incidentally, I can run a 208 AMAX out of that barrel at 2560 fps. with RL-17 and it will only have 72" of wind drift. Bullet and powder choice have a larger effect than barrel length, especially with the newer ones being offered. The longer barrel will only have an advantage if you optimize these for it or you add enough length to hit a higher node and your powder hasn't already attained a complete burn in the shorter barrel. The thing is most guys don't; they shoot a bullet and powder combination that makes their few inches of barrel length meaningless because it doesn't allow them to hit higher nodes than the guy with the shorter barrel. The guy with the shorter barrel can outperform them by simply being smarter about his powder and bullet choice.
Click to expand...

This follows what I have seen myself and heard from some damn good shooters.


My $.02 anyway.
 
Shooter ----/ said:
That is a very impressive velocity you've got there Kombayotch.
Click to expand...

I have 2 rifles in 308 in my own collection and my son shoots another that are all basically identical. All 3 we shoot 190 gr SMKs out of them and all 3 give velocities in the 2720 to 2745 fps area, all 3 have 21.75" long barre;s.

This is NOT all that uncommon when using todays newer powders. We found Vit 550 powder to 1 of the best for higher velocity loads with hevy bullets.
 
AlexF said:
I did detailled calculations using 175gr SMK at 2500fps in 16"
barrel and 2750fps in 24" barrel (experimental data).
The short answer is that you get the equivalent of 1% improvement in BC of bullet for each 1" of barrel from 20" up to 27" inches.
I would not extrapolate beyond these values since my velocity curved stopped there!

Barrel length Velocity 2680fps equivalent BC Drift 1000yards 10mph
---------------------------------------------------------------------
16" 2500fps 0.445 G1 (6% worse) 114"
20" 2620fps 0.465 G1 (2% worse) 108"
22" 2680fps 0.475 G1 (baseline) 105"
24" 2740fps 0.485 G1 (2% better) 102"
27" 2830fps 0.500 G1 (5% better) 97"

If all things are equal, a longer barrel gives higher velocity as long as the force from expanding gas (pressure * bullet surface) is greater than bullet-barrel friction.
Friction and pressure is pretty much constant near the muzzle since total barrel-chamber increases linearly and that friction is almost constant.

Again, not a quantum leap in performance but a 8" decrease in wind drift when increasing barrel length from 22" to 27" inches.

Alex
Click to expand...



Alex the problem with "experimental data and assuming all things are equal" is that in the real world they are not reliable in their conclusions.

I have tried many times to make identical rifles for competition shooters and despite the best efforts, it just does NOT work.

1 would think that when 1 had 2 barrels made from the same piece of bar stock, on the same day and both made by the same man on the same machines to the same specs, they would be identical. Right? WRONG!
I know because I did just that and the 2 rifles that I built from these 2 "identical" barrels shot and performed quite differently. To this day I am not 100% certain why they were notidenical. Possibly a variation of hardness within the steel bar stock the barrels were made of? Possibly the gun drill bit was ever so much slightly dulled? Possibly more pressure or less pressure was created by the hydralic oil being hot so the gun drilling machine operated slightly differently. Possibly 1 was lapped a billionth of an inch more deeply? Possibly the pressure on the cutter or the cutter was not as sharp when the rifling was cut on the 2nd barrel? Who knows?
But in the end a load that shot incredibly well through 1 rifle performed only mediocrely through the other rifle. Velocity differences were nearly 100 fps even with the idential load.

So given the variables 1 could deduce that there could be 100 fps more or less velocity if the barrels were not the identical lengths and would still be wrong.

Unfortunately there is alot more variation between guns than many like to admit. If 1 really wanted to get the "optimal" barrel length for a particular caliber with a particular load you would start off with a 36" blank, develop an accuracy load, then start cutting back the barrel incrementally until there was a velocity variation of note. The problem is is that you would be altering the harmonics with each cut, so then the accuracy node could be different.
In the end you would likley wear out the barrel in load development before reaching a concrete solution.

A close friend has another 308 I built with a 26" barrel on it. It will NOT shoot the 190 SMKs as accurately as many other of my rifles do and will NOT come close to the velocities my 3 rifles get with the same load. In that particular rifle 178 gr Amaxs seem to be the magic bullet and that rifle only gets them out at 2550 fps to get maximum accuracy. But with your theory that rifle "will" have less trajectory or drift, when in fact it does not.
 
Thanks for tip on the V550 Rick. I will have to find some and give it a try. I'm using a 20 inch barrel and am struggling to get 175 SMK's to your velocities in 190.
 
alberta tactical rifle said:
Alex the problem with "experimental data and assuming all things are equal" is that in the real world they are not reliable in their conclusions.

I have tried many times to make identical rifles for competition shooters and despite the best efforts, it just does NOT work.

1 would think that when 1 had 2 barrels made from the same piece of bar stock, on the same day and both made by the same man on the same machines to the same specs, they would be identical. Right? WRONG!
I know because I did just that and the 2 rifles that I built from these 2 "identical" barrels shot and performed quite differently. To this day I am not 100% certain why they were notidenical. Possibly a variation of hardness within the steel bar stock the barrels were made of? Possibly the gun drill bit was ever so much slightly dulled? Possibly more pressure or less pressure was created by the hydralic oil being hot so the gun drilling machine operated slightly differently. Possibly 1 was lapped a billionth of an inch more deeply? Possibly the pressure on the cutter or the cutter was not as sharp when the rifling was cut on the 2nd barrel? Who knows?
But in the end a load that shot incredibly well through 1 rifle performed only mediocrely through the other rifle. Velocity differences were nearly 100 fps even with the idential load.

So given the variables 1 could deduce that there could be 100 fps more or less velocity if the barrels were not the identical lengths and would still be wrong.

Unfortunately there is alot more variation between guns than many like to admit. If 1 really wanted to get the "optimal" barrel length for a particular caliber with a particular load you would start off with a 36" blank, develop an accuracy load, then start cutting back the barrel incrementally until there was a velocity variation of note. The problem is is that you would be altering the harmonics with each cut, so then the accuracy node could be different.
In the end you would likley wear out the barrel in load development before reaching a concrete solution.

A close friend has another 308 I built with a 26" barrel on it. It will NOT shoot the 190 SMKs as accurately as many other of my rifles do and will NOT come close to the velocities my 3 rifles get with the same load. In that particular rifle 178 gr Amaxs seem to be the magic bullet and that rifle only gets them out at 2550 fps to get maximum accuracy. But with your theory that rifle "will" have less trajectory or drift, when in fact it does not.
Click to expand...

Just to stir up the pot - 6.5 mystic - 4 barrels, 3 barrel makers (cut and button). Accuracy load within 0.3gr

260AI Shilen Select match - 2 barrels, Different times of manf. Loads within 0.1gr of the above 4 barrels. Same between barrels. Throated same as above barrels

All same lot components.

Similar results reported by various shooters in Canada, UK, Aus that I have chatted with on this chambering.

223 Shilen that I am using for my FTR - 2 barrels. manf should be similar in time but not sure if sequential. Same loaded ammo (within 0.1gr), same results - accuracy and speed.

Similar load and speed reported by other shooters using other brands of barrels.

Just depends on how you tune and what components you use. Is it an exact science? Of course not BUT it is possible to replicate results very closely.

DO NOT TAKE THAT AS LOAD DATA. ALWAYS WORK UP LOADS AS COMPONENTS CAN VARY WILDLY FROM LOT TO LOT....

But it sure makes it easier when you know what you are looking for.

Jerry
 
Well thanks all for the great posts. I think I have settled on 22". It's going to the smith on Friday to get cut down and threaded for my FTE brake. What kind of crown should I get done with the brake? Us there a special crown for rifles with brakes on? Thanks all.
 
Rotaxpower said:
I am thinking about getting my barrel shortened on my 700P in 308. The 26" barrel that's on it is a little long for my taste, and was wanting to get it shortened but not sure what length I should go with. What would you guys recommend? I was thinking about 20" or should I go shorter/longer? I also do plan some day to get out to 1000 with it just for fun not competition. Thanks all!
Click to expand...


Not sure if anyone has mentioned this or not and and since I'm too lazy to read all the comments, I'll take my chances and say it! Google 6mmbr, then go their .308 Win page. You will find a wealth of info there...the more important stuff has nothing to do with the loads, type of rifle or even barrel length!
 
alberta tactical rifle said:
I have 2 rifles in 308 in my own collection and my son shoots another that are all basically identical. All 3 we shoot 190 gr SMKs out of them and all 3 give velocities in the 2720 to 2745 fps area, all 3 have 21.75" long barre;s.

This is NOT all that uncommon when using todays newer powders. We found Vit 550 powder to 1 of the best for higher velocity loads with hevy bullets.
Click to expand...

RL-17 has a velocity edge over it. I compared the two a while back:
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1135095

Shooter ----/ said:
That is a very impressive velocity you've got there Kombayotch.
Click to expand...

Not that impressive anymore with the newer powders that have come out, and that wasn't the maximum velocity, just the node I run them at. Guys with 20" barrels are getting over 2600 fps at their maximum load: http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1299481


Some have even taken 220 gr. bullets over 2500 fps. out of a 20" barrel with RL-17: http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=2043155

I've been meaning to try it with the new Hornady 225 g. BTHP Match.
 
What pressure signs are you stopping at? I'm using W748 @ 45.2gr and primers are just slightly flattening. Sitting on top are 175 SMK.

Im using W748 because it appears to give me a full complete burn. I do not have bits of carbon scattered down the barrel. Now if I use RL 17, am I not still limited to the same pressures generated by W748? If I am, will this not translate into roughly the same velocity?

Thanks for the help. :)
 
I have pressure testing equipments, so I work up my loads in a test barrel and then I use the usual indicators when I move to my shooting barrels: stiff bolt, ejector marks, primers flattening and cratering. I run the load up to excess pressure levels in the test barrel to see where those start to happen with the specific components I'm using at the time. Be very careful when working up this kind of load since there is no specific order pressure signs will appear in. It's often different with different brands of primers and brass. Work up very slowly and stop if ANY pressure signs appear. Its no different than developing a load for a wildcat round, so use the same methodology and precautions.

Yes, you're still limited to the same peak pressure with RL-17. However, these powders will have different pressure curves. RL-17 generally has a wider pressure curve. It is the average pressure that the velocity is proportional to, not the peak pressure. That is a function of the area under the curve up to the time where the bullet leaves the barrel.
 
How loud could be a 18, 20 inch barrel for a .308w compared to a longer one.

I have see couples guys complain about this on different forum, but dont have the chance to test it myself. I have to say that my 30 inch barrel begin to be a pain in the ass.
 
Charles said:
How loud could be a 18, 20 inch barrel for a .308w compared to a longer one.

I have see couples guys complain about this on different forum, but dont have the chance to test it myself. I have to say that my 30 inch barrel begin to be a pain in the ass.
Click to expand...

A: the muzzle blast will be somewhat louder
B: the muzzle blast will be a few inches closer to you
 
Well it looks like I'm going back to my drawing board for my 308 :rolleyes: I have already picked up the VV550 and might as well grab a pound of RL17 and play mad scientist with my chrono!!!

I appreciate the help as I was a little concerned that I wasn't getting the velocity needed to stretch things out comfortably.
 
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