300 Mag reloading question

[igotta40]

I just bought a new Savage 110 Precision 300 Win Mag, and I started off setting up my seating depth using the Hornady modified case & comparator, to measure off my ogive to the lands. I started my baseline using a factory Hornady ELD Match 180 grain cartridge. 2.7” case head to ogive. Using the modified case and a 155 grain Sierra HPBT, I measured 2.9” casehead to ogive, touching the lands. So my difference between the two is 0.20”. That seems to me a lot of freebore. 2.7” vs 2.9” touching the lands. This sound right? Or is the difference in the two bullets?

 

[Blue68f100]

Thats the ogive profile of the bullet being different. One is more pointed than the other. Quite common. Depending on bullet some like a big jump 0.050"+, some don't.

 

[Chuck R.]

Thats the ogive profile of the bullet being different. One is more pointed than the other. Quite common. Depending on bullet some like a big jump 0.050"+, some don't.

Agreed, you're seeing the difference in the bullet profiles. Nothing to be concerned about unless that particular bullet doesn't perform with that much "jump".

Just to ensure I get as close as possible with my measurements I take 5 readings, then enter them into a spreadsheet that calculates the average and SD.

 

[Laphroaig]

Factory Hornady ammo is loaded to fit and function in all rifles and magazines so I am not surprised by the difference. There probably also is some difference in the ogive between the 2 bullets that contributes.

If the ogives are similar that is a huge jump, 0.200. Did you shoot any of the Hornady ammo?

 

[jmorris]

Or is the difference in the two bullets?

Sometimes is easier to see the difference if it’s drastic.

 

[Hugger-4641]

Yep, as already stated, its just the difference in two bullet shapes.
To each his own, but I approach it slightly different.
I use a dummy case with a slightly split neck and a bullet just barely seated, close this up in the gun, remove, then measure either the ogive or the OAL. Repeat 3 times to confirm. This is "jam" length for that bullet.
Then I back off 0.020 and start working down from there in 0.003 to 0.006 increments until I find what I want.

 

[243winxb]

https://saami.org/ drawing of chamber showes standard free bore.
.

 

[Laphroaig]

Yep, as already stated, its just the difference in two bullet shapes.

One measurement is of a factory loaded cartridge while the other is a Sierra bullet touching the lands of the OP's rifle; two different datums. The difference on the ogive also adds to the discrepancy but isn't the entire reason.

 

[igotta40]

Thanks for the replies. Maybe I’ll pull the bullet from the factory round and use it with my comparator and see what shakes out.

 

[243winxb]

Case head to bullet ogive measurement remains the same when bullets are jammed into rifling. Even with different bullets.

The Bullet Comparator Insert, at 308 contacts bullets at that diameter.