Calculating Freebore

[Jud96]

For awhile now I have been trying to mathematically figure out a way to determine optimal freebore for a given cartridge and bullet. Obviously the easy way is to send a dummy round to your smith or the reamer maker and be done. I however would like to figure out a way to accurately determine freebore length without needing anything more than numbers.

Berger has a really nice chart that lists their bullet specs and finding cartridge dimensions is easy online or in a loading manual. My original thinking was to start with the bearing surface length of the bullet. Let’s pretend here and say the bearing surface is 0.500”. Then I would look at the case neck length, let’s say it’s 0.300”. Now let’s say the lead angle is 1.5 degrees and it’s .020” long. I would simply add the lead angle to the neck length and then subtract that from the bearing surface length to get the freebore length needed to have the bullet seat right at the neck/shoulder junction. That number in this scenario would be 0.500 - 0.320 = 0.180”. So the freebore needed would be 0.180” plus say .030 to keep the bearing surface above the neck shoulder junction when seated off the lands. So my previous method would determine 0.210” of freebore would be optimal in this made up combination. However, I have found that often times my freebore calculation ends up being too long by say .050 or so. This can be a problem when COAL is important or if you’re trying to shoot multiple bullets in one chamber design.

My question is, how can I accurately determine freebore length within +-0.020 for any bullet and cartridge combo? It seems that I’m missing something and/or doing my calculations wrong. If anyone has any insight or knows how to do what I’m trying to do I would love to hear your thoughts and what you do. Thank you!

 

[DaveTooley]

You're leaving out a few numbers that matter. Consider that the FB diameter is larger than bullet diameter and that moves the engagement point further away. Simple trig to determine how much. Also the distance from the end of the neck section to the start of the FB cylinder section.

 

[Jud96]

You're leaving out a few numbers that matter. Consider that the FB diameter is larger than bullet diameter and that moves the engagement point further away. Simple trig to determine how much. Also the distance from the end of the neck section to the start of the FB cylinder section.

Thank you! Could you possibly give me an example so I can better understand? Thank you

 

[DaveTooley]

I could do the math for you but that wouldn't help much in the future. Look at SAAMI reamer specs. There will be measurements for the case/neck length from the case head and then the chamber length to the start of the FB. Then there is the difference between chamber/brass specs and actual case lengths. That's a third number.
For the trig part
freebore diameter-bullet diameter/ 2
That number divided by the tangent of the lead angle gives you the distance from the end of the cylinder section to bullet diameter.

I use my one Father's college math books of formulas and tables published in 1940.

 

[STS]

.......and then add a bit more to accommodate the barrel/bullet combo that likes .030 to .060 jump to shoot its best.

 

[hogpatrol]

1.5 lead angle is going to be longer than .020, more like .153 on a .004 depth groove.

 

[Jud96]

1.5 lead angle is going to be longer than .020, more like .153 on a .004 depth groove.

Can you explain to me how to determine that? That’s what I’m stuck on. Thank you

 

[hogpatrol]

Can you explain to me how to determine that? That’s what I’m stuck on. Thank you

Trigonometry Calculator

Angle A (lead angle) is 1.5, side a is .004 (groove depth). Assuming the difference between land and groove leg.

Where the bullet touches the lead could vary with ogive caliber.

 

[Mark W]

I’ve thought about this before as well. My limited math skills insure that I will have to keep sending my “dummy” round to the reamer maker....lol

 

[Jud96]

Trigonometry Calculator

Angle A (lead angle) is 1.5, side a is .004 (groove depth). Assuming the difference between land and groove leg.

Where the bullet touches the lead could vary with ogive caliber.

Thank you. I think another part I wasn’t thinking of was the angle of the ogive.

 

[DaveTooley]

I calculate it to the intersection with the groove diameter. That would be the longest point of contact. The reason is it isn't influenced by ogive shape. It's the intersection of a cylinder(bullet body) and the lead angle.

 

[Dusty Stevens]

I, like most gunsmiths, have a drawer or more dedicated to reamers that were a waste of money. Most were due to solving these type problems instead of going with what your gunsmith or reamer maker recommends. If your gunsmith cant figure out what you need there are plenty that have been around long enough to know. Different lots of bullets and different barrel bore dimensions can render a reamer useless in 6mo

 

[DaveTooley]

I'll add I know one reamer manufacturer who has tooling to measure that intersection. He has tooling -.001" under groove diameter for each caliber. That's his reference point. Worst case it may be a little short. More so with tangent ogive bullets vs secant ogives.

 

[Edd]

Thank you! Could you possibly give me an example so I can better understand? Thank you

Pick a cartridge and a Berger bullet and I will give you an example that will make it easy to get close.

 

[Jud96]

Pick a cartridge and a Berger bullet and I will give you an example that will make it easy to get close.

6mm-284 with a 108gr Elite Hunter

 

[Edd]

6mm-284 with a 108gr Elite Hunter

That is a wildcat. There is no standard spec for me to paste. It would be a lot easier for you to understand if you would pick a cartridge that has available drawings.

 

[Jud96]

6mm Creedmoor 108gr Elite Hunter

 

[Alex Wheeler]

One way to get close is to make up a dummy with the bullet seated where you want it. Lock a caliper .002" smaller than bullet diameter and spin the ogive against the jaw to scratch a mark on the ogive. Measure from the end of the neck to that mark. Next add the difference of the case length vs the chamber length. You will be close. I find theres still a little trial and error here even with sending out dummys. I have sent out a lot of dummy rounds to be measured and sometimes they reamer is perfect, sometimes not. Seems when they do measure it wrong its short, never too long. So once you get to know what numbers work, you stick with them.

 

[Jud96]

One way to get close is to make up a dummy with the bullet seated where you want it. Lock a caliper .002" smaller than bullet diameter and spin the ogive against the jaw to scratch a mark on the ogive. Measure from the end of the neck to that mark. Next add the difference of the case length vs the chamber length. You will be close. I find theres still a little trial and error here even with sending out dummys. I have sent out a lot of dummy rounds to be measured and sometimes they reamer is perfect, sometimes not. Seems when they do measure it wrong its short, never too long. So once you get to know what numbers work, you stick with them.

Thank you! It seems most of it is trial and error. I would like to be able to figure out the dimensions I need before ever making a dummy round or ordering a reamer, but it seems it may not always be exact.

 

[6ShotsOr5?]

Thanks to @Jud96 for the interesting question. I’m trying to follow this but a few pictures with some labels or a link to an article with some drawings would make it a lot easier.