Rebarreling from 308 to 243AI

[Capt Beach]

Traded into a 243AI barrel for my rifle...plan is to have a switch barrel that will function with my CDI DBM bottom metal and mags...barrel is a nearly new (less than 40 rounds) stainless fluted 26" threaded 5/8 x 24 off of a Savage 12 LRPV that was re-threaded to small shank and rechamber'd to .243AI. Its a 1:8 twist and I plan on shooting 115 Moly DTAC and Bergers into the 3100+ fps range. I have a few questions that I've gotten mixed answers to in my searching...

1. Head spacing, I've got 308 go and no go gauges. Good enough?

2. Fire forming brass...I know the best way to accuracy is new .243 brass that's been fire formed in my chamber on a properly head spaced gun...but buying 5 same lot boxes of premium .243 will get some trigger time in and should be fairly accurate AND yield fire formed "to my chamber" cases. Some say Lapua cases are worth the $$$ are they? I'll be reloading this rifle only other than donating standard cases to brass.

3. Experiences of others out there who have been there...done that...

4. Bullets and brass...any preferences here

This is the donor rifle...

 

[TEAMSENDIT]

fireforming works best with bullets jammed into the lands and factory ammo won't do that for you!!

in my 243ai 48.5grains of h4831sc, wolf primers, naked dtacs 3250fps 28" benchmark barrel

 

[280nosler]

Traded into a 243AI barrel for my rifle...plan is to have a switch barrel that will function with my CDI DBM bottom metal and mags...barrel is a nearly new (less than 40 rounds) stainless fluted 26" threaded 5/8 x 24 off of a Savage 12 LRPV that was re-threaded to small shank and rechamber'd to .243AI. Its a 1:8 twist and I plan on shooting 115 Moly DTAC and Bergers into the 3100+ fps range. I have a few questions that I've gotten mixed answers to in my searching...

1. Head spacing, I've got 308 go and no go gauges. Good enough?

2. Fire forming brass...I know the best way to accuracy is new .243 brass that's been fire formed in my chamber on a properly head spaced gun...but buying 5 same lot boxes of premium .243 will get some trigger time in and should be fairly accurate AND yield fire formed "to my chamber" cases. Some say Lapua cases are worth the $$$ are they? I'll be reloading this rifle only other than donating standard cases to brass.

3. Experiences of others out there who have been there...done that...

4. Bullets and brass...any preferences here

This is the donor rifle...

Head spacing with a 308 should work for a 243, but it won't provide an accurate measurement for the 243 AI due to the shoulder angle.

 

[Capt Beach]

Head spacing with a 308 should work for a 243, but it won't provide an accurate measurement for the 243 AI due to the shoulder angle.

You see that's where I come into problems...you have said that too...so where or what does the AI improved case head space on? Theres .1315" difference in the shoulders and I have not seen a 243AI head space gauge...and I will have no access to a fired case from this chamber...

So should I just head space it with the 308 go gauge or on a new live round and then fire form that round and re-head space it to check?

 

[former naval person]

The AI is supposed to headspace standard cartridges off the neck/shoulder junction, not at a predetermined datum line down the shoulder. I'd use a unfired case and check as you describe. By using the same brass you should be gtg. The next lot you buy may NOT headspace the same...but it is easy to fix in the Savage.

 

[Capt Beach]

I've just decided to buy a set of .243AI Head space gauges...I've been told that all I have to do is buy a 243AI Go gauge as the 243/308 GO gauge will function as a NO GO GAUGE for the 243AI...is this correct?

 

[280nosler]

The AI is supposed to headspace standard cartridges off the neck/shoulder junction, not at a predetermined datum line down the shoulder. I'd use a unfired case and check as you describe. By using the same brass you should be gtg. The next lot you buy may NOT headspace the same...but it is easy to fix in the Savage.

I don't know about the 243 improved - I have pulled out my PO Ackley Books to do some reading up, but there were a number that were not standardized off the neck shoulder.
Here is a link to 2 pictures (I don't know who owns the rights to the pics, so I don't want to save and post from my account), but you can clearly see the 30-30 improved is well above the neck/shoulder datum line, and there is a pic of the 243 and 243 improved below it as well. I wish I could answer the second part of the OP's question, but I do not know if the 308 would act as a no-go gauge, as it could be short (some of Ackley's cartridges were lengthened to the neck shoulder line.

 

[former naval person]

Anyway, standardized or not, if you have a medium touch fit on your new brass so that the firing pin strike doesn't shorten the head space, and the brass fireforms to the new chamber, you'll have a good fit...then worry about your dies. JMHO