Same case volume, same powder charge, right?

[blackhand]

Just wanted to get some feedback before working up a load with a new bullet. Bullet is an Acme 122 gr flat point. The thing is, I need to load the bullet very short to pass my EGW case gauge, 1.025" COL. At this length, just a hair of the driving band is above the case mouth.

 

At 1.025" COL, the seating depth of the bullet is .250". I checked a different load with an SNS 147 gr flat point bullet. That bullet I load to 1.125" COL and the bullet seating depth is .250". Remaining case volume after both bullets have been seated should be the same, right?

 

Can I use the same amount of powder safely in the 1.025" COL Acme 122 gr loads as the 1.125" COL SNS 147 gr loads since the case volumes are the same? Right now I use 3.6 gr WSF for the SNS 147 gr loads. It's a light target load. I understand the velocity of the two loads will be different. My main concern is that case pressure may be unsafe with a round loaded so short. On paper, seems like everything should work out safely, but I don't want to assume and miss something critical.

[ewthursby]

Your 147 powder charge is probably a good place to START for the 122. The 147 has more inertia and a longer driving band, so it takes less powder to create the same pressure. To get the same velocity out of the 122s you're looking at an increase of at least a few tenths of a grain. Your 122 seating depth is very short, so work up slowly. The smaller space means you will build pressure faster with a small increase in powder.

Take a look at your manuals and you'll notice that on average heavy bullets have lower powder charges than lighter bullets for the same powder.


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[ewthursby]

Your 147 powder charge is probably a good place to START for the 122. The 147 has more inertia and a longer driving band, so it takes less powder to create the same pressure. To get the same velocity out of the 122s you're looking at an increase of at least a few tenths of a grain. Your 122 seating depth is very short, so work up slowly. The smaller space means you will build pressure faster with a small increase in powder.

Take a look at your manuals and you'll notice that on average heavy bullets have lower powder charges than lighter bullets for the same powder.


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Sorry, I should have said to get the same power factor it will take a few more tenths of a grain. Velocity will obviously be lower with the heavier bullet.


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[NicVerAZ]

Do you determine that the case volume is the same by subtracting bullet length from overall length?

 

Just curious.

[blackhand]

1 hour ago, ewthursby said:

Your 147 powder charge is probably a good place to START for the 122. The 147 has more inertia and a longer driving band, so it takes less powder to create the same pressure. To get the same velocity out of the 122s you're looking at an increase of at least a few tenths of a grain. Your 122 seating depth is very short, so work up slowly. The smaller space means you will build pressure faster with a small increase in powder.

Take a look at your manuals and you'll notice that on average heavy bullets have lower powder charges than lighter bullets for the same powder.


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Ah, that's one thing I didn't take into account. So even if the bullet seating depth, case volume, and powder charge are the same for both loads, the 147 gr bullet will still generate more pressure due to greater inertia to overcome. 

 

58 minutes ago, NicVerAZ said:

Do you determine that the case volume is the same by subtracting bullet length from overall length?

 

Just curious.

 

Essentially. But I did: Overall length - case length. Take that and subtract it from the bullet length. That's how I calculated bullet seating depth for both loads to be the same at ~.250". Don't know why I made the extra step for myself.

[lgh]

(bullet length + case length) minus oal = seating depth.  Remember that you are probably using avg numbers and so depth, and therefore combustion volume, will vary as much as the averages do.  I usually will measure the length/weight/base of any new bullets just so see how much variation there is.  In my experience, the bullet length varies considerably more than the case length but it doesn't hurt to check case length too. Also, remember that your seating stem might be seating off the ogive or the bullet tip.  Ogive is better since most bullet variation occurs in the ogive, not the base, and seating off the ogive gives more consistent rounds since the bearing surface (bullet in contact with the bore) is more consistent and therefore velocity and pressure will be more consistent for our purposes. Also, do not assume that changing combustion volume (seating depth) varies pressure in some regular fashion. i.e. a small change in combustion volume doesn't necessarily change pressure by "just a little bit" or in any consistent way. 

 

Hope this makes sense and helps.  And check my math on seating depth to be sure it makes sense. And do correct me if I'm wrong. Always start low and don't hot rod loads unless you really know what you are doing.

[Southpaw]

8 hours ago, blackhand said:

The thing is, I need to load the bullet very short to pass my EGW case gauge, 1.025" COL. 

Get a new case gauge or just use your barrel. The EGW gauges suck with coated lead bullets because of this. You shouldn't be loading that short just to accommodate a case gauge. Get the hundo gauge, it's awesome and you won't have OAL issues like with the EGW.

[MemphisMechanic]

What Southpaw said. Load dummies really long (resize and crimp, just skip primer and powder) then shorten them up until they spin freely in the chamber. When they drop in and won't spin, you're touching the rifling and loaded too long.

 

Figure out how long your barrel will let you load (this will be different for every bullet weight and every brand) and then subtract .010"

 

Thats as long as you can load your ammo, and probably around the length you should be using. If your case gauge doesn't play nice with that? Sell it and buy one that does.

 

Eventually you'll end up with a 100rd shockbottle case gauge. Everything else is vastly inferior for making sure ammo is USPSA-grade accurate and precise.

[lgh]

Use your barrel own chamber for size check.

17 minutes ago, MemphisMechanic said:

What Southpaw said. Load dummies really long (resize and crimp, just skip primer and powder) then shorten them up until they spin freely in the chamber. When they drop in and won't spin, you're touching the rifling and loaded too long.

 

Figure out how long your barrel will let you load (this will be different for every bullet weight and every brand) and then subtract .010"

 

Do this for oal. It is oal for your barrel.

[blackhand]

Thanks for all the input, everyone. I made several dummy rounds, and the optimal COL for the 122 gr Acme bullet in my stock Glock barrels is 1.065". I do have one tight match barrel that requires loading down to 1.025" COL, but I was thinking of getting that one throat reamed.

 

I understand there's not a proportional relationship between chamber pressure and seating depth. I've seen cautionary figures that say even decreasing COL as little as .02" can double chamber pressure.

 

The cost of the Hundo gauge has prevented me from buying one for a while, but I may have to break down and buy one. I usually case gauge each round in my barrel rather than the EGW case gauge.

[MemphisMechanic]

1.065 seems EXTREMELY short for a factory Glock barrel. They normally eat ammo longer than anything else out there.

[Southpaw]

1 hour ago, blackhand said:

Thanks for all the input, everyone. I made several dummy rounds, and the optimal COL for the 122 gr Acme bullet in my stock Glock barrels is 1.065". I do have one tight match barrel that requires loading down to 1.025" COL, but I was thinking of getting that one throat reamed.

That's really short for a Glock barrel.  Try this test to determine max OAL for your barrel, 4th post:  http://www.czfirearms.us/index.php?topic=34225.0

If that's really the longest you can load that bullet in your barrel I would shoot up what you've got and then switch to a different bullet.  There are plenty of good coated lead 124gr bullets that can be loaded much longer in Glock barrels.

[blackhand]

Went back to the garage and loaded up a few more dummy rounds to figure out how I got those COL numbers. In the attached picture, the red bullets are the Acme 122 gr flat point bullets, the green bullets are Bayou Bullets 124 gr truncated cone bullets. You can see from the unloaded projectiles that the Bayou Bullets bullet has a longer taper and shorter driving band. Those bullets start to plunk and spin in a factory Glock barrel at 1.125" COL. The Acme bullets start to plunk and spin in the same barrel at 1.085" COL.

 

The Acme bullet loaded at 1.025" COL and the Bayou bullet loaded at 1.075" COL are rounds where the shoulder is loaded to 1/32" above the case mouth, based on 57k's information about SWC cast bullets. Both rounds at those lengths pass the EGW case gauge and chamber in my match-grade Glock barrels.

 

From reading around, the general suggestion is to then seat the bullet .005" - .01" deeper than the max COL to account for variations in case length, etc. I've found that in a lightly fouled barrel (50 rounds fired), bullets seated .005"-.01" shorter than max COL won't plunk and spin. To prevent rounds failing to seat and other problems in a dirty gun, I shortened the COL an additional .01".

 

So for the Acme 122 gr bullet, a COL of 1.065" will plunk and spin freely in a lightly fouled Glock barrel, 1.105" for the Bayou Bullet. I decided to try out the Acme bullets because in the last two cases of Bayou Bullets I've ordered, more than 10% of the bullets had visible seams where the mold comes together. Those bullets were not perfectly round, and you could feel a ledge all around the bullet with a fingernail. I had to load those bullets another .01" deeper for them to chamber in my barrels. The Acme bullets I'm trying out have no seams and look to be uniform in shape.

 

I prefer flat point bullets without a lube groove, and Acme and Bayou Bullets were the main options. The flat point bullets feed better through my bullet feeder, and I "feel" better about not having a lube groove.

 

I'll make a ladder for the Acme rounds at several COLs and see what works best. The Glock barrels I have don't seem to be too sensitive to changes in COL and shoot well over a range of cartridge lengths. Thanks for all the good information, everyone.

 

Edited February 4, 2017 by blackhand
Additional information.

[ArrDave]

If you are serious about case gauging, get a Hundo. Far and away some of the best reloading money you can spend


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[blackhand]

57K, Bayou Bullets still sells the a version of the bullet profile you were talking about in 124 gr. I never considered using a SWC bullet before, but now I'll have to.

 

If I ever get around to shooting USPSA, I'll have to break down and get a Hundo.

[anilson]

Get a new Case Guage, short ammo can have higher pressure and in most guns will not feed as well. We load .010 to .015 under the guns max OAL. This gives a buffer or variances.

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