.30-06 cartridge length

I’ve been setting the overall length of my reloaded .30-06 cartridges with .308” 150 gr FMJ-BT bullets to 3.250” as specified in my Sierra commercial reloading book. This cartridge length (3.250”) matches up well to the throat dimension (3.260”) of my 1917 Winchester M1917.

The test round I used to determine the throat length was an empty .30-06 case with a partially installed bullet. I inserted the test round into the chamber and gently closed the bolt which pushes the bullet deeper into the case when the tip encountered the rifling in the barrel bore. I then opened the bolt and removed and measured the overall length of the test round which was 3.260”.

I’ve been firing the same 3.250” ammo in my minty 1943 Remington 03-A3. However, using the same throat measuring technique mentioned above, I found the overall length of the test round is 3.323”. This is .073” longer than the M1917 cartridge. I then measured some .30-06 military ammo from the 1940s and 1950s and found that its overall length measured 3.322” which closely matches the throat length of the 03-A3 (3.323”).

The 03-A3 shoots the shorter ammo well but it makes me wonder if I should consider increasing the length of the reloaded cartridges that will be shot by this rifle to more closely match the length of the military ammo.

A good question might be - why is the M1917’s throat so much shorter than the 03-A3’s throat?

Only ever used 3.340" for all bullet weights. Save you a lot of mucking around.
"...throat so much shorter..." Every chamber is different in every rifle. Two P-17's will have a different chamber/throat length too. Throats wear out at different rates as well. Your '17 is 101 years old. The '03 is 75. 26 more years of wear and tear on the 17, assuming it has the original barrel.

Two different rifles made in two different places with the barrels chambered by two different reamers. .30-06 is not real sensitive to bullet jump, you can set your cartridge length for your shortest chamber and it'll work fine in your longest chamber. Getting a chamber length gauge, like Hornady's, might be a good investment. I've used the method you used and had some trouble getting consistent measurements. I finally figured out that the bullet was getting jammed in the lands enough that it would partially pull out of the case when extracting.

you got some room left with the 1917, move it out .005" and see how it works in both rifles.

Plus your not using the bullet that was the standard when both of your rifles were made. The 150 gr bullet was a flat base bullet and not a boat tail. The boat tail bullets are for the .308 Winchester aka 7.62x51. Most military rifles seem to shoot better with the flat based bullet too.