Gunsmithing How many grooves do I need?

[shoot4fun]

Barrels come with several different numbers of grooves (IE 3, 4, 6) and there must be a reason. I have a shot out 91/8 twist remington varmint contour in 243. I mean chunks are missing and the throat is toast shot out. It had a hard life on the prairie dog shoots.
I have a Hart 3 groove 14 twist currently chambered in 6PPC. It has 550 rounds and I am thinking of rechambering to 243 for this rifle. If I push the 55-70 gr pills thru it at warp speed will it stabilize in a 3 groove barrel?

 

[lvcatfish]

Re: How many grooves do I need?

You need at least one groove for each land...

Jokes aside that 14 twist seems a little slow (at least I have never seen a 14 twist in .243 bore. A friend has several 3 groove barrels that are serious shooters. I don't believe that # of grooves impacts stability so much as the selection of bullet weight/velocity and twist rate. Your bullet choice is on the light side of the caliber's possibilities so I expect this could work. Certainly there is a varmint addict here who can advise on the 4000fps stuff.

 

[former naval person]

Re: How many grooves do I need?

You'll do okay with the 55 to 70 grainers I believe. Ivcatfish is right on...the number of grooves shouldn't affect anything. Google "Greenhill's formula" and you can calculate a reasonably accurate result.

 

[shoot4fun]

Re: How many grooves do I need?

The barrel is a slower twist that is commonly used by BR shooters firing 6PPC. Most shoot the 66-68 gr bullets. I know I'll never make heavier work but I never hunt more than prairie dogs with a 243.
My 7MM08 does most of the bigger stuff.
Thanks for the input.

 

[buffybuster]

Re: How many grooves do I need?

During the 1930's & early 40's the US armories and weapons manufactures did a lot of testing (to reduce cost and increase productivity) and determined that 2-groove barrels shot just as well and lasted just as long as 6-groove barrels. The final conclusion was a barrel needed at least two grooves to stabilize the bullet.

Theoretically the few number of grooves will provide lower rotational drag since the bullet is not engraved/distorted by as many grooves. I don't know if that has any real world consequences even at LR, but it's a thought. Another school of thought is the few number of grooves means each land can be wider/larger and may resist erosion better than a barrel with more smaller lands. Again I am unaware of any empirical data on that but it is food for thought. With these considerations, a 3-groove barrel might be the ideal compromise.

 

[shoot4fun]

Re: How many grooves do I need?

Yup!
I thought I read about the military study and that some 1903 rifles were made with two grooves to save $$ and speed up production.
I'm looking around for a factory take off but if that doesn't work out I'll give this 3 groove barrel a run.
Thanks!

 

[buffybuster]

Re: How many grooves do I need?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: shoot4fun</div><div class="ubbcode-body">Yup!
I thought I read about the military study and that some 1903 rifles were made with two grooves to save $$ and speed up production.
I'm looking around for a factory take off but if that doesn't work out I'll give this 3 groove barrel a run.
Thanks! </div></div>

They ended up using that knowledge when they built those 2-groove Savage MKIV's and 03A3's during WWII.

 

[sbeckman]

Re: How many grooves do I need?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: shoot4fun</div><div class="ubbcode-body">Yup!
I thought I read about the military study and that some 1903 rifles were made with two grooves to save $$ and speed up production.
</div></div>

Yes. Smith-Corona, made a 2 groove barrel under a “War Emergency Steel” program.

From Wikipedia (FW<span style="font-weight: bold">T</span>W):
"To insure continued accuracy, these “M1903A3 rifles with two-groove 'war emergency' barrels were shipped with a printed notation stating that the reduction in rifling grooves did not affect accuracy".

There ya have it. 2 grooves is all you need.

 

[Brutas]

Re: How many grooves do I need?

The number of grooves is irrelevant. Usually, this is determined by ease of manufacture for a given rifling method.

The surface area ratio between the land width and groove width is what is important.