What benifit does a tight bore barrel have

[Guest]

I hear that the RockCreek barrels are tight bored and I see where Kreiger makes a tight bore barrel but I'm not sure what benifit they have over the regular bore barrels.
I know that alot of Palma shooters use them just don't know why
Do they boost velocity , help stablize bulltes that are long for twist. Are they only good in odd land and grrove barrels like 3 or 5 grove.

 

[Dave King]

JDJones75

My understanding of tight bore barrels is that they allow (force might be a better term) a tighter seal and less gas leakage. I believe polygonal barrels are also manufactured with a tight bore for this purpose. I have no proof of this personally, to small a sample for a real analysis I guess.

I do believe that the monometal bullets do not like to be squeezed too tight and the pressure may go high when using them in a tight bore barrel.

 

[rjb]

In regards to Palma barrels with "tight bore" they are/were come about by the competitors in International Palma matches
having to use a particular type of "ball 7.62
ammo" issued in UK more or less. The bullet
diameter was more like .307 than customary
.308 in USofA. Current NRA rules in U.S. allow Palma competitors to use 155 Sierras etc. that are .308 dia. The real "Tight Bore" barrels are changed to suit. There is some advantage in barrels over 30 inches long to have a (very) slight "choke bore" i.e. the chamber end starts out ~0.308 and at muzzle bore may be 0.3075. Takes skilled barrelsmith to lap that bore. rjb

[ 03-16-2004: Message edited by: rjb ]

 

[JR]

All depends on what ya'll mean by 'tight bore'...RJB is correct, palma barrel groove diameters were changed to .3065-.307" due to the RG 155's .307" OD...They dropped the bore size(land diameter) to .298 to give longer barrel life over standard .300(deeper lands to wear thru, as a barrel shoots out from chamber to muzzle generally)..When Sierra picked up the Palma, the bullets were and are running at .308", so groove diameters have followed suit in .3075-.308, but many are staying with the .298" bore diameter, to once again give better barrel life..Some are still shooting the .308 bullets in the .3065" barrels, but I wouldn't recommend this, as your bolt might get a bit sticky, ha..

Sometimes people use tighter groove diameters because they are using shallow throats, and don't want the grooves to shadow into the throat..

You don't necessarily get a tighter seal with a tighter bore, higher pressures maybe, because gas is going to escape around the corners of the land/groove intersection in conventionally rifled barrels..A tighter bore by itself in conventional rifling can lead to bullet upset as metal is not compressible, flows in length, so your bullet will get longer as mushes it's way thru the tight bore, and you will still get leakage round the corners..If you cant the lands a bit, or radius the intersection, or polygo-fy that gives you a better seal between bullet and bore..

That's my story and I'm stickin to it..ha..

JR

 

[chris matthews]

I'll back up JR, but Rock barels are 5R rifling not conventional. They don't deform the bullet as much since they have a land opposing a groove. And yes they will run faster than conventional barrels of the same length.