Brian,
My method of figuring this is pretty **** simple but it allows a comparision between the different land widths for speicific bore diameters.
First off, I figure the circumference of a specific bore diameter, lest say 7mm. The equation I use is 2(pi)r=c
So to figure this: 2 x 3.141592654 x 0.142= 0.892 rounded up.
Now I take and with a good light and my calipers, I get as close a measurement as I can of the lands at the muzzle. Just get as close as you can. YOu will easily be able to get within 5 thou of the actual land width with good light and possibly a magnifying glass.
Just for example I measured the following: THese numbers are just from memory as I do not have my notes in front of me now but you will get the point:
These are the widths of each land:
Rock 5R 1-8 twist(standard land width).....0.065"
Lilja 1-7 3 groove.........................0.075"
Lilja 1-9 4 groove.........................0.045"
Lilja 1-9 6 groove.........................0.025"
I then added the total wideth of the lands surface of a cross section of the barrel. Know I know that the lands are on a smaller circumference then the groove diameter and you could adjust for that if you wanted more accurate numbers but it is not that critical, I wanted to know the percentage of the circumference that was land width and compare those for each design.
If you take the Rock 5R barrel with 0.065" wide lands you come up with a total land width of 0.325. If you figure that into a ratio of the total circumference you come up with roughly 36% of the circumference is ladn width so it is pretty safe to say that 36% of the cross section of the bullet is compressed by lands.
If you look at the others you will find the following percentages:
Rock 5R...........36%
Lilja 3 gr........25%
Lilja 4 gr........20%
Lilja 6 gr........17%
Now the Rock 5R barrel was tearing the 200 gr ULD RBBT apart nearly all the time. Not the fault of the barrel, just not a good match with this bullet. In fact this barrel shoots well under 1/2 moa with the 160 gr Accubond with velocities near 3600 fps!!!
The Lilja 3 grooves have been working very well but on a couple occasions there have been instances where bullets have come apart with barrels that have gotten some wear on them. Again, not the fault of the barrel, actually more then a bullet baring surface compression issue it is more to high of RPMs more then anything combined with the wide lands.
In the Lilja 1-9 4 groove barrel, velocites pushing 3400 fps were possible and in the testing I was able to do never did I see any sign of bullet failure. All bullets made it to target predictably out to 1500 yards. THis is fully 250 fps more then what the Lilja 3 groove will allow but again, this is in large part to the overly fast twist rate come to find out we did not need such an extreme twist.
The Lilja 1-9 6 groove has the highest number of lands but also the narrowest and as such have been performing perfectly well.
The issues is largely twist rate but also land width as well.
The Rock standard 5R should not be overly hard on these bullets with a 1-8 twist at 2800 to 3000 fps which is terribly slow for the 7mm AM but we ever tested to this velocity and still had a very high rate of bullet failure.
Even the Lilja 3 groove with their very wide lands, widest of the group on average performed extremely well with this bullet because they only compress 25% of the cross section of the bullet.
The new Rock 5R barrel I am about to test which was made specifically for these bullets is a 1-8.7 twist, slower then the first one tested and the lands have been thinned to around 20 thou. Thats a bit over 11% compression.
Keep in mind though that the side of the 5R lands are not sharp edged like the Lilja barrel so there will be more compression then the land width predicts but still should be well under 20% which should work perfectly.
This is not an exact numbers figuring excersice, just one way to compare one rifling design to another and get a reasonably accurate comparision between the bullet cross section compression %.
With a set of calipers and a good light you can get extremely close to the actual land width of the rifling in your barrel.
TO test this theory I hope to get Rock to make me a 22 cal barrel with roughly 15% cross section compression and test it with the 100 and 107 gr ULD RBBTs in the 224 Allen Mag. This round has the potential to be a 3450 fps round in a 27" barrel with these bullets but so far the barrels I have tested will not allow this to be reached. Not even close.
Will need to talk with James at Rock Creek and see if a 22 cal barrel is possible. Thats a project for this spring though, to many irons in the fire for this fall and winter!!! /ubbthreads/images/graemlins/grin.gif
Did I clear anything up or make it alot worse!!! /ubbthreads/images/graemlins/grin.gif
Kirby Allen(50)