How do you determine the Free Bore length for a given bullet?
For example if I choose to use a 6mm Berger 108gr BT what should the free bore be?
For example if I choose to use a 6mm Berger 108gr BT what should the free bore be?
Free Bore vs Bullet
- Thread starter 500Stroker
- Start date
From the Berger Quick Reference Sheet, the base to ogive dimension is 0.678", and the boat tail is 0.217. Assuming you want to have the full bearing surface in the neck, you subtract the boat tail from the base to ogive dimension to get 0.461. From that subtract the neck length of your cartridge and that is what you need for freebore to touch the ogive to the start of the lands. That includes the length of the transition zone from neck to throat.
Alex Wheeler
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I can get pretty close with measurements. If I want it perfect I send a dummy to jgs and they nail it...
Why does free bore have to be perfect?
Most guns will change 1 thousand per hundred rounds and others even more .
Larry
Most guns will change 1 thousand per hundred rounds and others even more .
Larry
Alex Wheeler
Site $$ Sponsor
Its how some like things done.. It costs me no more to have jgs make my reamer perfect than it does for me to make a guess and tell them a number. I like bullet shanks up in the sized part of the neck, s and also wanting as much neck tension as I can get, and with a short neck like a dasher I get pretty picky about it. Also mag feeding. In the case of a 28 nosler for example, theres a fine line of too long to fit in the mag and the bullet being deeper than I want.
I know what we all like but I take barrel wear into consideration when getting a reamer . First few shots down the bore is where the largest Change is going to be . So what is perfect you guess or mine?
I have a gun with 133 fb It with 1200 round still is less fb then my 140 to start . The 140 fb has move to 156 with 800 rounds Same barrel company just using a different powder . Larry
Alex Wheeler
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Larry, never seen any throat movement in the first few shots... And yes throat wear plays a part. I like the bottom of the shank just above where the bushing stops sizing the neck.
A hundred is a few to me . Larry
I measured the neck length of a 6BRX case = .210
Subtracted NL .210 from 108 bullet dimensions .461 and ended up with a free bore of .251
Is that correct? It looks excessive.
Your comments please
Yes, that looks right to me, if you want the bearing surface to end at the bottom of the neck, and the boat tail to stick into the shoulder area. Also consider that this is not all free bore, but also includes any distance that the actual case neck is short of the chamber neck, and also the transition from neck to free bore. You will have to subtract those distances out to get the pure free bore.
..seems excessive to me ..most guys from what i have read are around 120 to 140 FB on BRX with 105-108gr ..i just did mine to 110 FB and will use this to FF once that is done i will be measuring my loads and working out how much i need to take out (if any to get my FB right)
just remember you can always take more material out to make FB more but you cant put it back in
using those figure to work out my socalled FB with 105gr hybrids i shout have .181 and i think that it to much but time will tell
i just saw your other stuff RonAKA about other measurements
so what would you say this works out as then ?
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Perhaps we are lost in terminology a bit. In your drawing I make the total freebore to be 1.838 minus 1.565, or 0.273. Seems to me it is more important to specify the dimension where you want the freebore to end, rather than just call it freebore.
There is also the issue of jump. If you know you are going to use large jumps like 0.080 to 0.100, you need to subtract that as well from the total freebore.
Thanks for help Guy's, I pretty sure I have a grip on it now.
Yep, depending on how you define terms, a throat of 0.060, lead of 0.013, and a freebore of 0.073". I was mistakenly adding in the taper into the lands. Strikes me that is not going to seat a very long bullet if you assume the ogive on the bullet stops at the end of the throat. And then there is the amount of jump you want.
carlsbad
Lions don't lose sleep over the opinions of sheep.
For JGS reamers, they have letters on each dimension. FB= U-V.
I order all my reamers now with long freebore for long bullets for long distance shooting. Please let me know if you have a 223 rem reamer with a .169" FB, known as the Kiff 223 ISSF. thanks, JERRY.
I order all my reamers now with long freebore for long bullets for long distance shooting. Please let me know if you have a 223 rem reamer with a .169" FB, known as the Kiff 223 ISSF. thanks, JERRY.
Consider that some gunsmiths order their reamers with a short throat, and then use a throat reamer to cut the throat deeper depending on the barrel twist and bullet lengths to be used by the customer. Saves them from keeping multiple reamers per cartridge.
The question about FB and bullet to me is simple .
Target shooting I want my FB long enough so I don't have to worry about the donut interfering with bullet seating .
Ever case develope donuts . The base of the bullet need to seat foward of that. Larry
Target shooting I want my FB long enough so I don't have to worry about the donut interfering with bullet seating .
Ever case develope donuts . The base of the bullet need to seat foward of that. Larry
tclaunch
Silver $$ Contributor
If in a 6 BRX and chamber length of 1.565" with 1 and 1/2 degree leade, I`d go .160" FB
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