What is the difference between a 298-3075 barrel blank and a 300-308 barrel blank
I see 22 (Centerfire) has this as well for 218/219 - 224.
I see 22 (Centerfire) has this as well for 218/219 - 224.
- Thread starter DocRDS
- Start date
Groove - bore diameter of the blank. First is known as a "tight" .30cal barrel. The other is a standard .30cal barrel.
The .298-.3075 barrel is great when shooting issued ammo where the bullet diameters are inconsistent or undersize. The throat may wear slightly faster.
If using normal quality bullets then there is no real difference in velocity, accuracy and precision.
The .298-.3075 barrel is great when shooting issued ammo where the bullet diameters are inconsistent or undersize. The throat may wear slightly faster.
If using normal quality bullets then there is no real difference in velocity, accuracy and precision.
Speaking in broad generalities, it's generally more conducive to precision for the bullets to be AT or slightly over groove diameter from what I've seen. That being said, I've seen plenty of .3078" bullets that shot amazingly. They still obturate and swell to the bore under the pressure load from expanding gas behind them so that's far from a hard and fast rule. I've shot .308 bullets in 7.62x54r before and while the rifle wasn't capable of incredible precision by modern standards, those .308 bullets shot just as good as .312" bullets, probably near the limit of what the system was capable of.
In general, I avoid tight bores because they do cause more heat and more jacket disruption, which can increase risk of bullet failures-- again that's a broad generality though. Lots of them are just fine with no issues even with super thin jackets. All of this is bell curve distributions and overlap and only the tails of the distribution crosses over thresholds so it's percentages of percentages-- hard to speak definitively. I would personally have less issues with tight groove diameter vs. tight land/bore diameter, but that's not based on anything scientific, just a hunch.
All else being equal (rifling sharpness, profile, count, ratio, etc...) half a thou on groove and 2 thou on the lands really shouldn't matter much at all unless you're at the far ends of spin rate, jacket thickness, pressure, velocity, heat, etc.
In general, I avoid tight bores because they do cause more heat and more jacket disruption, which can increase risk of bullet failures-- again that's a broad generality though. Lots of them are just fine with no issues even with super thin jackets. All of this is bell curve distributions and overlap and only the tails of the distribution crosses over thresholds so it's percentages of percentages-- hard to speak definitively. I would personally have less issues with tight groove diameter vs. tight land/bore diameter, but that's not based on anything scientific, just a hunch.
All else being equal (rifling sharpness, profile, count, ratio, etc...) half a thou on groove and 2 thou on the lands really shouldn't matter much at all unless you're at the far ends of spin rate, jacket thickness, pressure, velocity, heat, etc.
I once spent some time talking to Boots Obermeyer, who you might now from developing the 5R rifling barrel, and he made tight barrels at .298-.308 for sniper rifles. His barrels were used in the Mk11 and early M110 gas guns and used 168 grain 7.72 NATO rounds. His logic was that the tight barrel would squeeze and hold the bullet more consistently down the length of the barrel and out. He favored the 1:11.25 twist, which made me wonder if he might not have gotten the same result with a more open barrel and a tighter 1:10 twist. All I know is that if Boots said it was better, that is all I needed to know.
When we moved on to Bartlein barrels in gas sniper rifles, Bartlein cautioned against the tight barrel, as they felt it would wear faster. We ordered Boots' spec from Bartlein nonetheless, and got great results.
When we moved on to Bartlein barrels in gas sniper rifles, Bartlein cautioned against the tight barrel, as they felt it would wear faster. We ordered Boots' spec from Bartlein nonetheless, and got great results.
