OAL and CBTO

[cornfedkiller]

To make a long post short and answer my basic question, I recently learned about ogive, and without buying more equipment, I'm curious if I can use simple math to figure out the OAL for a new rifle. I have been shooting a 143gr ELD-X out of my 6.5CM. The OAL on my last gun was limited by the short magazine, and I was running my bullets at a OAL of 2.8125". I got a new gun and was loading up some bullets and it made me curious about the OAL to get closer to the lands. I talked to the guy I bought the gun from and he told me he measured the lands at 2.24", and he was running his bullets at 2.22" base to ogive. He was running a different bullet than I am, so obviously the OAL will be different.

Now I know the numbers differ a little from batch to batch, there's some room for error, etc... but I did some google research and they say the 143gr ELD-X bullet has an overall length of 1.43", and base to ogive is .673", so that tells me that ogive to tip is .757". If the base to .02" off the lands is 2.22, does that mean my new OAL should be 2.977"? That seems really long, but that's why I'm wondering if that's right? If I make one that long and it feeds into the chamber without any resistance, am I good to go?

I know the true answer is to buy a bullet comparator, but I'm just trying to figure out if I can figure this out without buying more stuff.

 

[rickjames80]

I use the saying "trust the math" fairly often, and it seems you could be onto something. However, if the original measurement came from a different bullet than I think you need to start over with finding the OAL for the ELDX.

I use a comparator, but I feel a guy can make some great ammo without one.

 

[mvrk28]

I would invest in a comparator. You’ll use it more than a lot of tools on your bench,

 

[Keats620]

X2 mvrk28's comment.

 

[ckleeves]

Are you just trying to find the lands? You can do that without any tools.


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[LaHunter]

CBTO is an important measurement when the goal is to maximize accuracy of your handloads. Rather than try to calculate it, just measure it and know for sure. The cost of a Hornaday Lock n Load bullet comparator kit is pretty low compared to the other equipment needed for handloading.

 

[cornfedkiller]

Are you just trying to find the lands? You can do that without any tools.


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Yeah I guess that's what I'm after so I know my OAL with this bullet. Like I mentioned in the original post, I know the length of the base to the lands, so I was just curious if I could use math to figure out a new OAL for this bullet.

Whats the best method for finding the lands without any tools?

 

[rhusby28]

You can buy a Hornady OAL Gauge for $35, and a cheap but very useable digital caliper for $25 or less. Modified cases are $7 a piece. I wouldn’t risk doing math when it’s super cheap to measure. Your loads will be more accurate and you won’t have to worry if your jammed into the lands or not.

 

[mvrk28]

Yeah I guess that's what I'm after so I know my OAL with this bullet. Like I mentioned in the original post, I know the length of the base to the lands, so I was just curious if I could use math to figure out a new OAL for this bullet.

Whats the best method for finding the lands without any tools?

There isn’t a best method, you need the tools. You need the comparator at a minimum. The problem with your idea of using OAL is that bullets are not identical. The distance from ogive to meplat can change from bullet to bullet. The only thing OAL is good for is knowing whether or not the loaded round will fit in a magazine. CBTO is 1000x more important.

 

[cornfedkiller]

There isn’t a best method, you need the tools. You need the comparator at a minimum. The problem with your idea of using OAL is that bullets are not identical. The distance from ogive to meplat can change from bullet to bullet. The only thing OAL is good for is knowing whether or not the loaded round will fit in a magazine. CBTO is 1000x more important.

Makes sense. So you guys that are using comparators, are you measuring every round individually? Press the bullet in, measure the CTBO, then do the same with the next bullet? Or just once per batch of bullets?

 

[huntsman22]

Before all these comparators, Hornady cases and tools were readily available, guys would just use a bullet and cleaning rod. Put a flat end jag on the rod, drop in in the bore, making sure it is flat on the bolt face without being affected by ejectors and such. Close bolt, mark rod at muzzle. Next, drop a bullet into the chamber and hold it against the lands with a pencil or such. Put rod back in bore and mark it at same place on muzzle as before. Measure that distance and seat that particular bullet (since they vary) to your length. As long as your die seating stem is not seating using the tip, you can adjust depth from there, then lock your die down and go to loading......

You can use the same method and use a comparator or Sinclair hexnut to get the ogive measurement, but the results end up the same. There are dedicated rods available with included stops now. Drop the rod to get boltface measure, lock the upper stop. Get the bullet-on-lands, lock the lower stop. Measure between stops. easy, peasy.....,.

 

[Downeaster]

Huntsman has it, the world of accurate seating depths didnt start with hornady. Another cheap/free method you can use is to make a slice with a cutoff wheel, down from the mouth into the junction of the shoulder. Takes a little experimentation to get the tension right (i squeeze with my fingers), but keep sliding in a bullet till you can get it to move with medium finger pressure.You can also slightly kiss the mouth with the seating die. Another method to achieve the same goal is taking a fired case that you give just a shade of neck tension and inserting a bullet and wiggling side to side by hand to decrease tension. Leave the bullet a little long and chamber. Repeat multiple times to check for consistency. If you are seeing scuffs from the lands you are too tight. If you are getting longer lengths than you started with it means you are gripping the chamber and will want to increase tension a little, but that depends on the bullet ojive and alloy as well (experimentation is key here).
Remember to remove the burrs from the cutoff wheel Inside the neck.
This may not be the best method, but since you were trying not to spend money on a comparator, i figured id give you another option.

Best of luck.

 

[mvrk28]

Makes sense. So you guys that are using comparators, are you measuring every round individually? Press the bullet in, measure the CTBO, then do the same with the next bullet? Or just once per batch of bullets?

In all honesty I think it depends on the quality of components you are using. For my primary rifle I’m using a Redding Type S FL bushing die and a Whidden Gunworks Micrometer seater, they are very consistent so I only check every 10 or so and they’re usually always the same CBTO. But I’ve also got some cheaper stuff and those I check every round because they tend to vary. At first I would check every round and then once you feel it’s consistent check every 5 or 10.

 

[mahnster]

Makes sense. So you guys that are using comparators, are you measuring every round individually? Press the bullet in, measure the CTBO, then do the same with the next bullet? Or just once per batch of bullets?

Check with the same frequency you would check your OAL. I check 100% for the first 5 rounds or so to make sure it is consistent and then reduce to 1 out of 10.

 

[bpotter]

Ive switched to finding lands by seating a bullet long and trying to close bolt on the bullet, keep seating bullet deeper until it closes as and there is no click when you open the bolt. Measure with a caliper and Sinclair or other bullet comparator when done and work your bullet depth off this number.

Depending your action, you may have to take a few parts of the bolt to avoid interference. It helps to have a micrometer seating die to speed up process.

Do a search on wheeler accuracy how to find your lands exactly for a video.

COAL is only good for interpreting load data and length bullet your mag will and action will cycle.

 

[rhusby28]

Makes sense. So you guys that are using comparators, are you measuring every round individually? Press the bullet in, measure the CTBO, then do the same with the next bullet? Or just once per batch of bullets?

I generally measure the first two or three. When I’m happy with the results I seat the rest. A micro seater die helps with adjustments. Forster and Redding both make a great micro seater. I also seat a bullet into an empty case so I have a reference when I want to make a new batch of rounds.

 

[Sawfish]

If you have a magazine fed gun, then your magazine will dictate the OAL of your loaded rounds. You can use one of the methods above, or the method used by J.D. Jones of SSK fame for single shot firearms. Simply seat the base of the bullet even with the base of the cartridge case neck. Works surprisingly well for many different cartridges,

 

[BRTreedogs]