COAL and dummy rounds

[snod85]

First post. Thanks to all who actively participate on these forums. I've spent the last several months reading hundreds of threads, using the search function, and trying to learn. Absolutely love the community and have greatly benefited as I move from a Dillon 550 loading pistol and mil-spec rifle ammo to precision reloading for target ammo.

I have two rookie questions. Question 1. How important is it to have the dummy round COAL match the actual round?

The listed COAL for 6mmbr is 2.44" and 2.80" for 6x47L. My brother and I are having rifles built for 1:7.5" and 1:12' twist barrels in 6x47L as well as 1:13.5" and 1:10" barrels in 6mmbr.

Another member posted a video years ago of the proper way to give your smith dummy rounds where you seat the heel of the bullet to the neck/shoulder junction. After rough measuring (laying the bullet out beside the case) I found my COALs for 6mmbr ranged from 2.03"(Sierra 70 Blitzking) to 2.24" (Nosler 90 Spitzer) using this method. The COALs of the 6x47L varied from 2.36"(Barts 68 Ultra/Berger Column) to 2.61"(Sierra 107 Matchking/105 Scenar).

I understand that free bore comes into play here and am requesting 0 to 0.045 FB for the 13.5" 6mmbr as it will be for 60-70 grain bullets, a 0.060-0.080" FB for 1:12 and 1:10 twists for both 6mmbr and 6x47L in 65-90 grain bullets. Finally 0.105-0.120 FB for 6x47L with 80-107 grain bullets. All free bores are based on the most recommended option/range from previous threads.

Should I be concerned in the COAL variation between bullet types and a consistently shorter COAL than what manuals lists? The Sierra manual does adjust the COAL to the bullet and is very close to what I measured.

question 2. Is the % fill of powder listed in some bullet manuals based on a bullet heel to shoulder/neck junction seating? Does it stop at the neck/shoulder without considering bullet heel or base location?

Thank you.

 

[Ned Ludd]

Question 1) The problem using cartridge overall length (COAL/COL) measurements is that bullet nose length can vary widely, perhaps as much as .015" to .020" (or more) within a single Lot# of bullets. Cartridge base-to-ogive (CBTO) measurements tend to be more uniform/reproducible, and therefore may be more useful during the reloading process.

Does using COAL versus CBTO measurements really matter for the purpose of a dummy round to be supplied to the gunsmith? Perhaps it's not so critical, but I'd still try to get it right. The dummy rounds you provide to the smith should be spot on with respect to where you think you will start seating a specific bullet in a brand new chamber. I personally like to have a bullet seated in the case neck such that the boattail/bearing surface junction is at least 1/4 of the neck length above the case neck/shoulder junction but no more than about halfway out the neck. In this scenario, the bullet ogive should be at an optimal distance relationship to the lands (i.e. optimal seating depth) closer to the 1/4 the way out the neck end. That will allow the most room for moving the bullet further out in the neck to maintain a certain ogive distance to the lands as the lands erode over time. Most (but not all) case neck lengths about one caliber long, so having the bullet boattail/bearing surface junction (i.e. the bottom of the bearing surface) from 1/4 to 1/2 the way out the neck means there would be from 0.5 caliber to 0.75 caliber worth of bullet bearing surface seated in the neck. It also means you have about 1/4 or so of the total neck length to play with in terms of seating depth and land erosion.

This is also critical to consider: if you don't have any idea where the bullet might want to tune in in terms of optimal seating depth, you can hedge your bets and make the dummy round somewhere in the middle. For example, you could make the dummy round with the bullet boattail/bearing surface junction seated about 1/3 of the neck length above the case neck/shoulder, or perhaps a tick more. This would give you a little more room to move the bullet farther away from the lands when optimizing seating depth, but still leave some room to seat it further out as the lands erode. As I noted above, 1/4 of the total neck length is approximately what you have to play with. So for a .223 Rem cartridge you'd have approximately .050" to play with, and about .075" to play with for a .308 Win neck. Thus, use this window to your advantage in the dummy round to give yourself some room to adjust seating depth, as well as to account for land erosion over the life of the barrel.

FWIW - the bottom/base of the bullet boattail will most likely still be seated below the neck/shoulder junction with the boattail/bearing surface seated at 1/4th the way out the neck, possibly even 1/2 the way out the neck. That all depends on the relative length of the bullet boattail and is why I would rather use the boattail/bearing surface junction as the indicator, rather than the base of the boattail; the amount of bearing surface in contact with the case neck wall is the critical aspect here, not the relative boattail length. I have crudely illustrated the approximate region I want the bullet boattail/bearing surface to be seated at below:



Just be aware that none of this is written in stone. I have seated bullets with less than 0.25 caliber worth of shank in the neck, and with the entire bearing surface seated well below the neck/shoulder junction. So placing the boattail/bearing surface junction outside of the region I indicated above can work just fine in the right circumstance. Sometimes one may be forced to do it if the chamber has already been cut in order to use a bullet of certain dimensions. But it's usually not optimal and you have the opportunity to do it right at this point in the process, so I would suggest discussing it carefully with your smith, then doing as careful a job with the dummy rounds as possible.

Question 2) The powder fill ratio will depend on factors that include bullet seating depth, as well as internal case volume. Unless the actual internal case volume is measured, any fill ratio is likely to be a rough estimate at best. This is particularly true when different brands of brass that may have different case wall thickness and therefore different effective internal volume are used. Reloading programs such as QuickLoad or GRT can generally provide better (although not necessarily perfect) fill ratio values because they require bullet dimensions, case volume, and loaded cartridge dimensions as necessary inputs.

 

[RegionRat]

I have two rookie questions. Question 1. How important is it to have the dummy round COAL match the actual round?

Once you hand your gunsmith a dummy round and ask them to cut a chamber to touch the lands of this round, you are committing to those dimensions. In other words, yes it is very important. You can also ask for some jump for the dummy round, but again, that is a committment.

If you are noticing similarities between bullets in loading manuals, it is because the ogive is similar.

question 2. Is the % fill of powder listed in some bullet manuals based on a bullet heel to shoulder/neck junction seating? Does it stop at the neck/shoulder without considering bullet heel or base location?

Those fill ratios are typically based based on their COAL numbers and virgin brass. So it does consider their bullet recipe at their seating depth.

Other than to avoid unintentionally getting into compression loads, don’t get too hung up on the fill ratio numbers in manuals. Unless you are following their recipe using that exact same brass and seating depth, those numbers can vary.

That is not to say it isn’t wise to watch out for when a fill level will cause seating depths to get into compression, but learning when your recipe is difficult to seat or when it might cause runout or require the use of special powder settling techniques is very important.

 

[CJ6]

If your Smith chambers for Match or Target rifles, ask him for some advice on what he'd do to meet your needs. COAL listed in reloading books is more about magazine length than anything else. If you shoot single shot it 's what you want/need it to be.

 

[snod85]

Thanks gentlemen.

The original plan (per advice on here) was to submit dummy rounds which was, as I understand it, primarily for the smith to insure the chamber diameter was cut allow for a .003" minimum clearance with my particular brass. As I am using Peterson brass I am a tad concerned the slight differences between it and Lapua make this important. Also would like to not exclude V-max bullets, which may be slightly fatter (also from comments on here) than the target bullets. My smith is really good and experienced so I trust him to keep me between the ditches.

I did note the bearing surface on most bullets allow for seating depth changes that could reach the listed COAL and still retain a bullet diameter of seating depth on most/longer bullets. The shorter/lighter bullets being the exception but if a 1/4 diameter is acceptable that works too. I guess seating up in the neck is where the "doughnut" comes from.

Seating the dummy round bullets using 1/3 of the bearing surface as along as it does not exceed listed COAL makes sense to me. Still leaves room for final adjustment. Need to remeasure/refigure. My hope is for the dummy rounds to provide my smith additional data points other than chamber dia., not to adjust the chamber dimensions solely on those rounds.

The fill question is because I'm reluctant on compressed loads and note that Peterson brass loads seems to be a little less than Lapua based on comments here. Initially I will be a lower node type of guy, at least until I get my feet under me.

All my reloading thus far has been to SAAMI spec so first time thinking about these issues..

Thank you for responding. Very helpful.

 

[J.D.Erwin]

Question 1) The problem using cartridge overall length (COAL/COL) measurements is that bullet nose length can vary widely, perhaps as much as .015" to .020" (or more) within a single Lot# of bullets. Cartridge base-to-ogive (CBTO) measurements tend to be more uniform/reproducible, and therefore may be more useful during the reloading process.

Does using COAL versus CBTO measurements really matter for the purpose of a dummy round to be supplied to the gunsmith? Perhaps it's not so critical, but I'd still try to get it right. The dummy rounds you provide to the smith should be spot on with respect to where you think you will start seating a specific bullet in a brand new chamber. I personally like to have a bullet seated in the case neck such that the boattail/bearing surface junction is at least 1/4 of the neck length above the case neck/shoulder junction but no more than about halfway out the neck. In this scenario, the bullet ogive should be at an optimal distance relationship to the lands (i.e. optimal seating depth) closer to the 1/4 the way out the neck end. That will allow the most room for moving the bullet further out in the neck to maintain a certain ogive distance to the lands as the lands erode over time. Most (but not all) case neck lengths about one caliber long, so having the bullet boattail/bearing surface junction (i.e. the bottom of the bearing surface) from 1/4 to 1/2 the way out the neck means there would be from 0.5 caliber to 0.75 caliber worth of bullet bearing surface seated in the neck. It also means you have about 1/4 or so of the total neck length to play with in terms of seating depth and land erosion.

This is also critical to consider: if you don't have any idea where the bullet might want to tune in in terms of optimal seating depth, you can hedge your bets and make the dummy round somewhere in the middle. For example, you could make the dummy round with the bullet boattail/bearing surface junction seated about 1/3 of the neck length above the case neck/shoulder, or perhaps a tick more. This would give you a little more room to move the bullet farther away from the lands when optimizing seating depth, but still leave some room to seat it further out as the lands erode. As I noted above, 1/4 of the total neck length is approximately what you have to play with. So for a .223 Rem cartridge you'd have approximately .050" to play with, and about .075" to play with for a .308 Win neck. Thus, use this window to your advantage in the dummy round to give yourself some room to adjust seating depth, as well as to account for land erosion over the life of the barrel.

FWIW - the bottom/base of the bullet boattail will most likely still be seated below the neck/shoulder junction with the boattail/bearing surface seated at 1/4th the way out the neck, possibly even 1/2 the way out the neck. That all depends on the relative length of the bullet boattail and is why I would rather use the boattail/bearing surface junction as the indicator, rather than the base of the boattail; the amount of bearing surface in contact with the case neck wall is the critical aspect here, not the relative boattail length. I have crudely illustrated the approximate region I want the bullet boattail/bearing surface to be seated at below:

View attachment 1455877

Just be aware that none of this is written in stone. I have seated bullets with less than 0.25 caliber worth of shank in the neck, and with the entire bearing surface seated well below the neck/shoulder junction. So placing the boattail/bearing surface junction outside of the region I indicated above can work just fine in the right circumstance. Sometimes one may be forced to do it if the chamber has already been cut in order to use a bullet of certain dimensions. But it's usually not optimal and you have the opportunity to do it right at this point in the process, so I would suggest discussing it carefully with your smith, then doing as careful a job with the dummy rounds as possible.

Question 2) The powder fill ratio will depend on factors that include bullet seating depth, as well as internal case volume. Unless the actual internal case volume is measured, any fill ratio is likely to be a rough estimate at best. This is particularly true when different brands of brass that may have different case wall thickness and therefore different effective internal volume are used. Reloading programs such as QuickLoad or GRT can generally provide better (although not necessarily perfect) fill ratio values because they require bullet dimensions, case volume, and loaded cartridge dimensions as necessary inputs.

Ned
That is some very good info and well explained and you just proved the OP post there's good information and people on Accurate Shooting forum.

 

[snod85]

Yep, I am printing Ned's reply and take to the reloading room, remeasure, and think more on it.

 

[Webster]

First post. Thanks to all who actively participate on these forums. I've spent the last several months reading hundreds of threads, using the search function, and trying to learn. Absolutely love the community and have greatly benefited as I move from a Dillon 550 loading pistol and mil-spec rifle ammo to precision reloading for target ammo.

I have two rookie questions. Question 1. How important is it to have the dummy round COAL match the actual round?

The listed COAL for 6mmbr is 2.44" and 2.80" for 6x47L. My brother and I are having rifles built for 1:7.5" and 1:12' twist barrels in 6x47L as well as 1:13.5" and 1:10" barrels in 6mmbr.

Another member posted a video years ago of the proper way to give your smith dummy rounds where you seat the heel of the bullet to the neck/shoulder junction. After rough measuring (laying the bullet out beside the case) I found my COALs for 6mmbr ranged from 2.03"(Sierra 70 Blitzking) to 2.24" (Nosler 90 Spitzer) using this method. The COALs of the 6x47L varied from 2.36"(Barts 68 Ultra/Berger Column) to 2.61"(Sierra 107 Matchking/105 Scenar).

I understand that free bore comes into play here and am requesting 0 to 0.045 FB for the 13.5" 6mmbr as it will be for 60-70 grain bullets, a 0.060-0.080" FB for 1:12 and 1:10 twists for both 6mmbr and 6x47L in 65-90 grain bullets. Finally 0.105-0.120 FB for 6x47L with 80-107 grain bullets. All free bores are based on the most recommended option/range from previous threads.

Should I be concerned in the COAL variation between bullet types and a consistently shorter COAL than what manuals lists? The Sierra manual does adjust the COAL to the bullet and is very close to what I measured.

question 2. Is the % fill of powder listed in some bullet manuals based on a bullet heel to shoulder/neck junction seating? Does it stop at the neck/shoulder without considering bullet heel or base location?

Thank you.

The published COAL is most likley so it will fit in a magazine and nothing to do with getting some amount of jump. Your gunsmith doesn't need a dummy round. No reason to seat the bullet heal at the neck/shoulder junction for a standard. Ask the gunsmith what you need for freebore for the bullet wt. you want to shoot. All your recommended freebores look very excessive to me. Ask a gunsmith that competetes at a high level on this website like Wheeler or Bart ect. Try e-mailing a big name barrel maker, tell him what bullet you want to shoot and ask what he recommends for FB. To many opinions on this website by amatuers. My 6BR has 0.030" freebore for 60-68 gr bullets, I should have got zero freebore but it probably doesn't matter it's a GH hunting rifle and it shoots small. The gunsmith already had the 0.030 FB reamer and I didn't want to pay for a new one. I shoot almost all bullets with 0.010" jump to touch the rifling. 60-80 thou FB for 60-70 gr bullets is ridulous.

With 0.030" FB and 0.010" jump bullets from 60-68 gr are only half way down the neck. A 55 gr bullet is only about 0.040" into the neck. With 0.010" jump some of your FB numbers the bullet probably wouldn't even be in the neck. you have to decide on an approximate bullet wt. you will shoot and get the proper FB from an expert. And not from some keyboard expert.