Making 7mm-08 from 308 vs 243

[JPB4BAMA]

There are a couple of old threads on this, but I would like some fresh opinions.
I'm toying with building a 7mm-08 AI for midrange BR and F open. Like the idea of SPR and the means likely having to convert brass from 308 or 243 SPR brass. So just looking for thoughts on which route would be best, assuming that the answer is there is a difference in approach. As a background, I live on the gulf coast and not likely to experience cold weather ignition problems that some mention with SRP in this cartridge, and I don't typically chase the hot loads, thus the 7-08 and not a 280 or 284, (AI is more for brass stability than case capacity)
Any thoughts are appreciated.

 

[XTR]

Personal opinion here, but I don’t think AIs of 308 based cases are worth the effort. The case has very little taper to start with. I seem to recall about a 4% max increase in capacity. When you consider that a 30-06 gains about 150 fps over a 308 with a lot more capacity ya can’t expect much. As always, what you do is up to you, I’m just some guy on the WWW.

As for brass, I shoot WW 7-08. If you want to make it right now I’d pick what is available.

If you use 308 you will almost assuredly need to turn because the necks will get thick.

If you use 243 or 260 you’ll likely get a donut, not a problem if you can seat far enough out to keep the bearing surface ahead of it.

I don’t know if it’s easier to run it over a mandrel or through a die, probably equal effort.

personally my hunting rifle runs 140 class bullets (ABs and SGKs) at 2950 from a 25“ barrel with 2000MR.

 

[gunsandgunsmithing]

Necking up creates a donut and necking down thickens the brass...but both ways can work fine. I think I prefer necking down if given the choice and not turning necks.

 

[XTR]

I’ve never made it, if he necks down WW its a couple of tho thinner and I can see it being OK with a big enough neck on the reamer, but if you use SRP, that’s Lapua/Peterson/Alpha brass. Can you get away w/o turning?

Now I may go try it with a scrap Lapua case. my chamber has a .317 neck.

 

[gunsandgunsmithing]

I’ve never made it, if he necks down WW its a couple of tho thinner and I can see it being OK with a big enough neck on the reamer, but if you use SRP, that’s Lapua/Peterson/Alpha brass. Can you get away w/o turning?

Now I may go try it with a scrap Lapua case. my chamber has a .317 neck.

Neck a piece down and seat a bullet, then measure it. You need at least .002 clearance. If you don't have it, you can either turn necks or have the neck opened up.

 

[butchlambert]

Mine are made on 308 Lapua. I fireform without a bullet after necking it down. This way I have no donut and mine have not been neck turned.

 

[HandgunHTR]

Having experimented with my 7-08AI for a few years now, I have found that there is no appreciable gain in using SRP brass over LRP brass.

I necked down 308 brass for experimenting with SRP brass and ended up having to ream the necks 0.001-0.002". I went with reaming over turning the necks due to personal preference (makes less of a mess), not because I think it is better. I tried regular SRP, BR SRP, and SRMPs.
For regular LRP brass I used Remington, Lapua, and Starline. LRP and BR LRPs. Where I have settled is regular LRPs and Lapua brass. I still use the Remington for hunting as my groups are still at MOA with the same load (they are about .5MOA with the Lapua brass). That being said, as it looks like we won't be seeing Lapua 7-08 brass for a while, I will be switching over to Starline here soon.

 

[Laurie]

Having experimented with my 7-08AI for a few years now, I have found that there is no appreciable gain in using SRP brass over LRP brass.

I'd agree, in fact got smaller groups and SDs with LRP Lapua than with necked-down 308 'Palma' cases (with Viht N160) in 'straight' 7mm-08. The latter allow higher-pressure loads though as the small primer brass case-heads are very robust. A couple of 'odd' results showed up in a side by side comparison in that unlike 308 LRP v SRP, I saw little to no loss of MV with N160 and in fact on a range of charges saw higher MVs with SRP for the two highest charges. With the then newly introduced Viht N555, SRP results were consistently poorer across a range of charges, so as always SRP + powder compatibility needs to be checked with this size of case / charge weights.

In a 'minimum-SAAMI' 7-08 chamber, I was a tad concerned about neck-clearances with reformed 308 Lapua as it started around 14 thou' thickness and some rose to 15 or a little more in necking-down. I have a lot of very thin / light 1980s Norma 308 brass (the so-called 160gn cases) some of which I reformed too for a mildish short-range load, but they were used straight out of the die having started out at 12.5-13 thou' neck thickness, and this would no doubt apply to Winchester and R-P brass too.

 

[butchlambert]

I just measured the necks of my Lapua 308 necked to 7 08 AI once fired. .0136

 

[Laurie]

I just measured the necks of my Lapua 308 necked to 7 08 AI once fired. .0136

Lapua 308 neck thickness is pretty variable from lot to lot IME. This particular lot of 'Palma' 308 were all pretty well at 14 thou' to start with and had a few with individual highest readings (of three taken per case around the neck) of 0.0145", or even a fraction more. So, it's well worthwhile to check after reforming.

 

[gunsandgunsmithing]

Lapua 308 neck thickness is pretty variable from lot to lot IME. This particular lot of 'Palma' 308 were all pretty well at 14 thou' to start with and had a few with individual highest readings (of three taken per case around the neck) of 0.0145", or even a fraction more. So, it's well worthwhile to check after reforming.

Alright Laurie....I love you BTW! So by your numbers, we need a .3375 chamber to have essentially zero clearance with a .3085 bullet...Right? So, we need at least .002" clearance, which gives us a .3395-.3400 chamber, minimum. Are we in agreement, or essentially, to this point? I prefer .003" but isn't this workable? I'm just not seeing the problem but admittedly, maybe I overlooked it elsewhere. I do think most match no turn 308 reamers are a tad bigger. I think around .343(lapua)but I'm very open to correction on that. The ones I've done have been hammers with .3435 neck diameters.

Help me out. I'm probably overlooking something. Thanks!--Mike

 

[SteveOak]

Personal opinion here, but I don’t think AIs of 308 based cases are worth the effort. The case has very little taper to start with. I seem to recall about a 4% max increase in capacity. When you consider that a 30-06 gains about 150 fps over a 308 with a lot more capacity ya can’t expect much. As always, what you do is up to you, I’m just some guy on the WWW.

As for brass, I shoot WW 7-08. If you want to make it right now I’d pick what is available.

If you use 308 you will almost assuredly need to turn because the necks will get thick.

If you use 243 or 260 you’ll likely get a donut, not a problem if you can seat far enough out to keep the bearing surface ahead of it.

I don’t know if it’s easier to run it over a mandrel or through a die, probably equal effort.

personally my hunting rifle runs 140 class bullets (ABs and SGKs) at 2950 from a 25“ barrel with 2000MR.

A 243 case is 53 grains of water. A 243 AI case is 58 grains. 9.43%.

Another point in favor of the AI is the sharper shoulder which results in less powder to be pushed into the barrel before it burns. This results in better efficiency. If you "Ackleyed" the case and then shortened it to have the same case capacity as a 243 you would get more velocity out of it.

To the OP, 260 Rem brass is pretty close to 7-08.

 

[Laurie]

So by your numbers, we need a .3375 chamber to have essentially zero clearance with a .3085 bullet...Right?

Ahhh .... this is a topic about 7mm-08 brass. So, we're talking 0.284 + 0.015" + 0.015" + desired clearance, say 0.317 or 0.318". PT&G one of whose reamers cut my 7mm-08 chamber lists both standard and match versions as 0.316". So with 15 thou' neck thickness brass after reforming, the result would fit but to my mind be on the tight side. (@butchlambert 's 0.136" reformed neck thickness brass ends up at 0.311", so would be a good no-turn fit.)

So far as 308 Win goes, I've never had the slightest worry about Lapua brass in a 'minimum SAAMI' chamber irrespective of lot and resulting thickness. In my FTR days, I'd measure every neck on new cases at three points and if I felt variations were on the large side, (there again being some variations between production lots in this metric), would do a minimum 'clean-up' neck-turn. Whether that gained me anything worthwhile is highly debatable, but the reassurance is psychologically nice!

 

[gunsandgunsmithing]

Ahhh .... this is a topic about 7mm-08 brass. So, we're talking 0.284 + 0.015" + 0.015"

Ha! Told ya I was overlooking something.

 

[dryhumor]

I had sized a 260 to 7-08 the other day as an example for the OP. Went ahead and did two more as I was shooting the 7-08 this morning.

The Peterson 260 loaded neck with 154gr SST was .313 across three rounds. The Alpha 7-08 once fired, same bullet, was .312 across ten rounds. My chamber is .317. 154 SST is .2845.

260 new was 2.020, fired plus .003. I can’t see, feel, or measure a donut. I haven’t resized yet.