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First off BR barrels are not slower because they are tighter.
I think that is a misconception stemming from the tight bore Palma rifles which are 30" and set up to only shoot 155s. They are the only barrels set up that way normally.
BR rifles use standard match barrels.
Now there are some barrels that will be faster due to the type rifling, which gives less pressure with a standard charge which means you can go to more powder and get more MV.
These include canted rifling types such as the Rock 5r, Broughton 5C and any other with this type rifling. It is widely known that most Broughton 5C barrels will run about 50-75 fps faster than standard rifling in same twists.
Also twist rate will be a factor. For example, most people use the old standby of a 1-10 in a 30 cal. That barrel will develop higher pressure and lower MV than a 1-11 or 12 twist with the same 300 win mag shooting 210s as the 1-10 spins the bullet faster and gives you a higher pressure. The 1-10 was the standard for heavy bullets at lower velocities. When you up the MV (such as a 3006 compared to a 300 Win or 300 WSM) you can go to the slower twists with less RPM, and still shoot the same heavy bullet faster.
Seirra says that you need a 1-9 for their 240, yes if you shoot it at the lower velocities. Yet if you take a 300 Weatherby (no free bore either), 300 Ackley or 300 Rum for example, they will all easilty shoot the 240 and very accurately as they are much faster and do not need the faster twist to stabilize the bullet.
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You are aiming your comments at the incorrect person.
Second stability will be a factory of MV. As MV slows loss of stability will be a factor. 240s can easily go in excess of one mile accurately in the right gun and case combo.
Most of the BR bullets are Flat base with a pressure ring that is larger than the bullet diameter. The game is pure accuracy, most have no clue what their MV really is.
I shoot the BIB FB bullets that have pressure ring, I give up some in BC compared to the VLDs. However, in the pure accuracy game, BC is minor compared to higher quality FB bullets. I do not have to sort by ogive, I do not even Juenke my normal match bullets. I will for the Nationals though. Plus I can go one twist slower, right now I am shooting a 1-12 with 300 WSM and 187s (3025 fps) and a 1-9 with 105/108 Bibs in a 6mm BRDX (AI) (3130 fps).
Bullets will vary in diameter by mftr. Krieger has a .236 and a .237 bore diameter for its 6mm barrels. Different bullets for different barrels. However, they are the main mftr that does this and only in 6mm to my knowledge. That is mainly due to the short range BR game.
Palma guys use the tighter bore because the rules say 155s and 308s to 1000 and they need the MV to get there. That combo works for them.
bottom line it normally is a combo of twist, bullet weight/design and MV.
In my experience, a barrels velocity potential has very little to do with its bore diameter alone. There are many factors to figure into the equation and so many variables that its nearly impossible to predict which barrel will be faster then another.
Things such as number of lands in the bore, wideth of the lands themselves, throat diameter, throat length, lead angle, all of these things will combine to result in the velocity of a barrel far more then just the diameter of a bore.
Of course bore surface quality also comes into play as well.
I have seen exactly the same barrel design from a top end barrel maker produce nearly 100 fps difference in top velocity potential.
To figure out exactly why this happens is more likely to give you grey hair then actually conclusions because your theory in most cases will be weakened with every barrel you test down the road.
I have also found that actual twist rate will effect velocity potential very little comparing slower twist rates with faster twist rates. That said, the most dramatic differences I have tested with velocity change in relation to twist rate has been with some of my more extreme Allen Magnum wildcats.
I should correct that statement by saying, pressures are effected more by twist rate in my opinion moreso then velocity is. This is what I mean by that.
In my 7mm AM, I can take a very long 200 gr ULD RBBT in a 1-7 twist 3 groove barrel that is 30" long and get 3300 fps comfortably with 102 to 103 grains of WC872 on average.
I can also take a Lilja 1-9 twist 6 groove barrel, also 30" and get that same 3300 fps just as comfortably but it will take 105 to 106 grains of the same powder to reach this velocity.
What this tells me is that while the twist rate does not really seem to limit the velocity potential of this combination as far as fps produced, the twist rate does effect how many grains of powder are needed to get X fps.
To be really fair, this comparision should be with either two 3 groove barrels or two 6 groove barrels, not one of each!!
Still, in my opinion, bore diameter does very little to effect velocity potential in and of itself.
Allen Precision Shooting
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