Gunsmithing  Building dummy rounds to give to gunsmith?

BurnOut

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I am in the process of transforming a somewhat unusual gun, a T/C Icon. It is currently chambered in 7mm RM, and I want to get it re-barreled to .338 WM. Because the rifle has an internal box mag (Remington BDL-style) with an internal dimension of 3.500", I want to have my chamber/throat cut such that I can get close to (and chase, if necessary) the lands with my projectile(s) of choice while still allowing for feeding out of the the somewhat stubby internal mag.

I rolled into Cabela's the other day, and bought a box of off-the-shelf 338 WM ammo... and have come across a used set of dies that I can use to assemble the dummy rounds after I (bullet) hammer the projectiles out of the cases and dispose of the powder. The idea is to take the new cases, get a hold of a box of the projectiles that I want to use (Berger 250 grainers), and put together some rounds with the COAL that I want... then providing those rounds to the 'smith and asking him to chamber my barrel to match. Pretty good plan, right? Right.

Anyhow, here's where I run into a question: what can I do to get the shoulder of my cases moved out to meet SAAMI chamber specs? I'm asking because sometime back, I compared some off-the-shelf FGMM loaded ammo (in .260 Rem) to a Forster GO gauge (using the Hornady case comparator set) and found that the cases on the FGMM loaded rounds were, on average, .007" short of SAAMI length at the "datum" line ("datum" is in quotes because I don't know that I actually measured to the datum line, only that the point of measurement between the FGMM cases and the GO gauge were the same because I used the same insert from the case comparator set).

Is it a situation to where I can ask that the chamber be cut to SAAMI dimensions, while the throat is cut to accommodate the ogive of my projectiles at my desired COAL? This is my first go 'round with this sort of thing, so I admit that I may well be overthinking it... thanks for any help/advice.
 

Ledzep

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    You're getting into weird territory where either you, or your gunsmith need to know exactly what's going to (or is supposed to) happen. .338wm is a belted magnum. The belt is what the headspace is traditionally set on, not the shoulder. If you want the shoulder to be the headspace point, you're going to need gauges made to set it to a specific value.

    Most people don't go through that trouble... typically it's just set off of the belt with an appropriately designed reamer, and you adjust your dies to set the case head space on the shoulder (or a slight "bump"). HOWEVER, doing so may not properly size all of the case that needs to be sized, and certain areas-- namely just forward of the belt-- may remain unsized by the die, and can cause case-sticking-after-firing issues.

    With all of that sorted--- Yes, usually the dummy round is made of resized or virgin brass with the desired COAL, and is used to set the throat length while head space is set with head space gauges.
     

    BurnOut

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    Ledzep- I appreciate your response. I *had* thought about the belt, and about how it might impact what I'm trying to do, but your post confirms it. I have heard (well, read on the Internet, so you know it's true) that configuring belted cartridges to headspace off the shoulder often results in accuracy improvements... but I don't know how much truth there is to that.

    I'm thinking of getting a set of custom dies made, sending in some fired cases to the manufacturer (will probably go with Whidden) to hopefully address the issue with resizing above the belt.
     

    Garyc

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    Anyhow, here's where I run into a question: what can I do to get the shoulder of my cases moved out to meet SAAMI chamber specs? I'm asking because sometime back, I compared some off-the-shelf FGMM loaded ammo (in .260 Rem) to a Forster GO gauge (using the Hornady case comparator set) and found that the cases on the FGMM loaded rounds were, on average, .007" short of SAAMI length at the "datum" line ("datum" is in quotes because I don't know that I actually measured to the datum line, only that the point of measurement between the FGMM cases and the GO gauge were the same because I used the same insert from the case comparator set).

    Is it a situation to where I can ask that the chamber be cut to SAAMI dimensions, while the throat is cut to accommodate the ogive of my projectiles at my desired COAL? This is my first go 'round with this sort of thing, so I admit that I may well be overthinking it... thanks for any help/advice.

    You don't need to move your shoulder out on the dummy cases to give to the gunsmith. Belted magnums headspace off the belt, not the shoulder. If you look at a forster GO gauge for a belted magnum you'll see it doesn't even have a shoulder, it's only the belt part because the shoulder is irrelevant. You couldn't use your hornady comparator to compare a belted magnum go gauge to the unfired brass like you did with the 260. Where the shoulder ends up in the chamber is a function of the gunsmith's reamer, it's built into the profile. He'll be setting the headspace off the belt though, not the shoulder. After you've fired your brass in that chamber you can choose to use the shoulder for headspace, which is what I do with my belted magnums, but a gunsmith can't set the headspace off the shoulder and have it be to SAAMI specs because SAAMI means it headspaces off the belt.

    As for assembling the dummy rounds, remember that the max COAL for a 338 win mag is 3.340" which means your 3.500" magazine box is actually pretty long for it so you have some room to play with the bullet seating depth. Bergers are long bullets but you should still be able to seat the 250 grainer to a reasonable length and stay within the 3.500" constraint. I would seat the bullet to where the junction of the boattail is right at the shoulder/neck junction of the case and measure it. So long as it was less than 3.500" (minus a little slop, say .050" so 3.450" or so) I'd tell the gunsmith that was my desired seating length and to throat it to touch the lands at that length.

     

    ajwcotton

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    One other thing, aside from what everyone else has said, please make sure your dummy rounds have plenty of neck tension and very low run out. It is very difficult to throat a chamber with sloppy dummy rounds. It usually ends with the customer being unhappy. Either the Smith has to try to fix the ammo, or get a old of the customer to have them send a better dummy, which they may not have components to or knowledge to make.
     

    BurnOut

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    All- I appreciate the input. What I ended up doing, after building my dummy rounds, is using a Hornady OAL comparator setup to check the base-to-ogive length of my dummies (using the Berger 250 EH) vs. some factory ammo (Hornady 225 SST). Despite a shorter OAL, the factory ammo was .020" longer from the base to the ogive than were my dummies. Because I want to be able to use factory ammo in a pinch, I sent three pieces of the factory ammo to the 'smith along with my barrel, and asked him to throat it to those cartridges. I may as a result end up with a throat that's slightly longer than ideal for my preferred projectiles loaded at magazine length, but what I'll gain in return is the flexibility to use ammo other than my hand loads if the need arises. Additionally, my skill as a shooter is such that even if there is, say, a .2 MOA improvement in group size to be had by throating the barrel to my dummies, I wouldn't be able to consistently see it anyhow, as that's within my margin of error behind the trigger.