throating 7mmRemMag for 180 eld-m

[Edd]

To calculate the exact length to land contact takes some information other than just a measurement with a comparator bushing. You need to know the hole diameter of the bushing and if it isn't real close to the bullet diameter, within .001" or less, you need to know the angle of the bullet nose.

 

[AKSavage]

To calculate the exact length to land contact takes some information other than just a measurement with a comparator bushing. You need to know the hole diameter of the bushing and if it isn't real close to the bullet diameter, within .001" or less, you need to know the angle of the bullet nose.

I measured the hole diameter of my two comparators when this first started out, since I was getting two different numbers. The holes apparently taper in a little inside the hole, so I measured as close to the edge of the hole as possible, about 1/16" in. My Sinclair comparator measures right at .284, while my Hornady comparator measures closer to .274. This jives with the Hornady comparator giving a longer OAL, since it hits the taper of the bullet earlier. A bullet that measures .284 using the same micrometer gets marked right at the beginning of the body when I jam it into the Sinclair comparator, from either the ogive or the boat-tail end. All indications are that the Sinclair comparator should be pretty close to actual values.

 

[desertcj]

I have the Sinclair "Hex" comparator that has different calibers.

 

[Edd]

At a length of 2.818", the bullet will contact the lands with a freebore length of .268". This measurement is calculated using the SAAMI length of 2.5398", a freebore diameter of .2845", and a throat angle of 1.5°.

 

[AKSavage]

Looking at a 7mmRM reamer print with a .114 throat, the lands start at 2.534". Looking at a reamer print with a .179 throat the lands start at 2.699".

Something is wrong with your first set of numbers. The case is 2.500". Add .114 and your at 2.614". The bottom numbers are close but I'm still at 2.679" with a .179" throat?

Yes you are right. I was making a mistake. The number I was reading on the reamer print was not to the bolt face, so it would NOT be the OAL of the cartridge. Good catch. It's to some datum line IN FRONT of the bolt face, on both reamer prints. The top of the belt, maybe? It does not say how far in front of the bolt face this datum line is though, although almost all the other measurements are referenced off this line. The problem is that neither reamer print shows HOW FAR from the bolt face this reference line is, or it would be a simple matter to add this in to get the OAL, and hence subtract the same amount from the .295 throat length. Time for some more calculating and head scratching. Maybe someone more versed in reamer prints can chime in. Both reamer prints I'm looking at are from PT&G, and in both cases the numbers/measurements (2.534 and 2.699) are the closest to the edge of the paper.

 

[AKSavage]

At a length of 2.818", the bullet will contact the lands with a freebore length of .268". This measurement is calculated using the SAAMI length of 2.5398", a freebore diameter of .2845", and a throat angle of 1.5°.

OK, I have it now, and my numbers match yours. .268" of freebore it is. First of all, looking closer at both reamer prints the reference line IS marked at .22" from the bolt face. Take the distance marked from the reference line to the end of the neck and add .22" and I get 2.5398". And looking at a SAAMI drawing, the reference line is at the top of the belt.

For those interested, the trick is to add in the distance it takes for a 1.5 degree angle to taper .00025" off of each side, reducing the diameter from .2845" down to .2840".(.00025/tan(1.5)=.0095") So, take 2.5398 and add .0095" to get 2.5493". Subtract this from 2.818 to get .2687" of freebore.

 

[AKSavage]

knowing where the 180 sit in my 7saum and adjust for different neck length .263 should be what your looking for

Yup, that's very close to what we came to agree upon as well. Thanks for your input, it was spot on.

 

[desertcj]

My math was off....I was measuring off of the dummy rounds I sent Shilen.

 

[can1010]

Yup, that's very close to what we came to agree upon as well. Thanks for your input, it was spot on.

good luck on your build
180 eld-m shoot real good from .010 off to +.002 in lands