The 708 & 708AI: Trails & Tribulations

[BFD]

2020AD BFD became aware of 7mm projectile proliferation in the competition sector due to high BC...
2021AD BFD R&D discovered the 708 cartridge, with 120HH bullet, out of a bolt gun, extinguished the souls of critters like the wrath of an angry god...
2022AD BFD completed the build on a 708 AR10 rifle and extinguished the souls of critters as a demi-god walking upon the crust of the Earth...
2023AD BFD took the 708 to the next level with 708AI match reamer. @aratliff joined the project and two identical Benchmark 7mm blanks were turned into 708AI barrels... (I was shooting for a Terminator Franchise tone with all that... )

So, anyway, me and Aratliff played twinzies on these 708AI barrels and ARC Nucleus actions. I sent our barrels to Patriot Valley Arms to have chambered with the 708AI reamer I got from JGS. PVA did a spectacular job. The barrels came back and I was first to load develop with Alan agreeing to bird dog my process once perfected. This is where the story gets interesting...

I made 708 brass for the 708 bolt gun and gas gun by running 308 FGMM brass through a 708 FL die. Worked and stills works perfect and is very easy to do. So, I did the same process with Lapua brass for the 708AI rig. Those 308 brass sized to 708 brass shot phenomenal out of the 708AI barrel during fireforming to get 708AI brass. What I just typed can confuse, so:

Virgin Lapua 308 brass is run through 708 FL die.
Newly formed 708 brass is fireformed to make 708AI brass
708AI brass then gets 1-2grains more powder added to reach full nuclear capabilities of 708AI*** (*** = STUPID)

Here are the first 15 shots of 708 brass for fireforming:

My sweet 708AI Rig:


As you all can imagine, I was one heel clicking, happy AF, sombuscuit! So, I shot a whole lot of the 44.5gr H100V load to re-enforce I was the World's best shot, and to fireform the brass to 708AI.

After a few hundred cases of 708AI brass formed, and after numerous awards in the International Sniper Competitions, F-Class Nationals, bwahahaha just kidding, I only needed two hundred cases....

Ok, seriously.

The problems started when I shot the fire formed 708AI brass. I added 1-1.5gr more powder and shot the 708AI rounds for group. Nightmare! They were shooting 3MOA groups. Erratic and made me crawl into the fetal position and suck my thumb. Naturally, I tried some different charge weights and disaster struck. WIN 760 & H100V kept things in check, but RL17 @105F. heat index blew the project all to hell. After another volley of W760 & H100V rounds printing Helen Keller groups, I shot my RL17 46gr loads and the 3rd shot went KABOOM!

I pulled the trigger and immediately I felt pressure to my face, & my shooting hand. I actually uttered out loud, "WTF"! Upon immediate inspection I saw the bulged magazine a foot away, and the rounds that were within the magazine strewn about. I haven't shot with protective eyewear for anything in 20+ years and this event changed that. My eye was watery and sensitive for a couple weeks. I could have easily lost my right eye with all the debris shot about at high velocity. Thank God I got lucky and didn't loose my shooting eye.

After a lot of investigation into this incident it was found that:

1. I had loaded a RL17 charge that went over pressure in the heat
2. I had donuts form in the 708AI formed brass
3. The brass that kaboomed came back from a cross cut analysis that showed fissures in the case web.
4. Chamber neck clearance was zero inches. A 7mm bullet would not fit/slide into a neck of a fired case.

Conclusion for cause of kaboom:

1. Rare factory flaw of a case web
2. A catastrophic pressure event with the combined neck donut and zero case neck chamber clearance. Pressure blew rear of cartridge and gun.
3. Match chamber reamer specs that does not allow for the thick necked .016 case necks of Lapua brass

Good news is I have control over conditions 2 & 3 with case neck turning. Bought the K&M neck turning kit that is ran with the case chucked into a cordless drill:

The K&M neck turner takes out the case neck donut and thins the case neck walls from .016 to .013 which allows a proper chamber neck clearance. The rounds loaded for the chamber dimensions created by the match reamer now fire problem free with laser beam precision!

 

[Farleg]

Gday bfd
Great write up & glad you were ok but a reminder to all on the need for eye protection which I’m still guilty of not doing all the time but try to change my old habits

That kaboom thread caused some friction on lrh but the gentleman ( it was bob correct ? ) that did the cutaway tests should be commended for his great work & one other gentleman for also pointing out what to also look for a few others they can go -oot their boot for all I care

moving on
Great trigger work & I love that 7-08 & most likely stay with it as I just don’t get the fireform process ( just personal nothing against it just not me & I’ve only done one lol )

So my question is what do you see the advantages of going to the AI & would you do it again if you had your time over again
Less the kaboom of course

Cheers

 

[BFD]

Gday bfd
Great write up & glad you were ok but a reminder to all on the need for eye protection which I’m still guilty of not doing all the time but try to change my old habits

That kaboom thread caused some friction on lrh but the gentleman ( it was bob correct ? ) that did the cutaway tests should be commended for his great work & one other gentleman for also pointing out what to also look for a few others they can go -oot their boot for all I care

Yes, Bob Wright is who did the cut away test and filled me in on the pressure soaking of the fire forming. Talked and text with him for a while, he's a really good dude.

moving on
Great trigger work & I love that 7-08 & most likely stay with it as I just don’t get the fireform process ( just personal nothing against it just not me & I’ve only done one lol )

So my question is what do you see the advantages of going to the AI & would you do it again if you had your time over again
Less the kaboom of course

Cheers

Fordy, outside of being a very sexy looking round, I'm not seeing but 50fps advantage in the 708AI over the 708. Not anything like the 150fps advantage when I did the .243AI. That .243AI is a cartridge that really benefits from the AI effort. I will continue with the 708AI due to being heavily invested into the 708AI brass, custom reamer, neck turner kit. I can process this brass fairly quickly due to the lessons learned from my goat show rodeo.

 

[aratliff]

After the above referenced incident, I put the project on hold. Following a FUBAR with the post office where I didn’t receive the neck turning equipment, I punted on the 308 brass route and ordered regular 7mm-08 bras for fireforming. Hoping that saves a step and stays within the clearances of the chamber. My plan is to pick it back up after the first of the year.

The project has been full of joy, but also full of pain and frustration…to say the least! Kudos to Brad for sticking with it and also for not being too proud to share the snafu’s and mistakes along the way.

 

[joe16]

Glad you didn't get hurt

 

[Mountainman]

That's a beautiful rifle amigo. It's frustrating as hell when things don't go according to plan, in fact I've changed my plans to include a week's worth of cussin and pouting during load workup so I'm not disappointed . Glad you weren't hurt and I'm sure you'll get that thing sorted out.

Been thinking of another 7mm build myself just can't figure what I really want.

Regards

 

[BFD]

That's a beautiful rifle amigo. It's frustrating as hell when things don't go according to plan, in fact I've changed my plans to include a week's worth of cussin and pouting during load workup so I'm not disappointed . Glad you weren't hurt and I'm sure you'll get that thing sorted out.

Been thinking of another 7mm build myself just can't figure what I really want.

Regards

Thanks! I have the load process figured out using Alan's twin barrel to finish up load development. Same barrel, reamer, etc so I perfected the load development and sent Alan back the barrel and the recipes. He will be able to hit the ground running with his 708AI

Why did I finish up with Alan's barrel and not my own? Now get this. A second tragedy struck me the very same day as the kaboom. After I got back from the range I let wipeout accelerator sit in the bore and it rapidly ate the barrel like I poured acid in it. 45min it was toast. $1200 barrel ruined by Wipeout products. The owner of Wipeout denied that the accelerator could hurt barrel steel. He rattled on about everything else in the world that caused the issue, but maintained the accelerator could never hurt steel. When the charlatan stated I was the first person to ever have a problem with Wipeout accelerator, I caught him in a flat out lie. I had researched the issue and found this occurred in the barrels of a gunsmith that had 7 barrels ruined. When I reminded him of that incident his reply was, "What, are you looking to win the lottery?" I replied "absolutely not trying to win anything, I just lost a ton of money by using your product." His next response was I would have to hire a good attorney and sue him. Yeah, absolute dirtbag.

Juxtapose this company treatment with Benchmark Barrels...Benchmark absolutely wanted my barrel sent in to them for analysis. The rep said they certainly want to know if their steel was at fault and are all about keeping their quality control on point. They conducted the analysis and stated they didn't find anything bad with the steel, but said they will send me a new 7mm blank! Man, can't get any better than that!

 

[jrebel]

Glad you’re not hurt….sounds like a scary shot for sure. I’m sure you also ruined a pair of underwear that day, but understand why you didn’t add that part to the story….

In my humble opinion….AI’s don’t offer much benefit given todays modern bullets. If I wanted the extra speed out of a 708, I would drop to a lighter bullet that will still crush game. Point being….I’m shootings the 101gr HH out of a 20” barreled 708 (savage youth rifle), at 3150 fps. It will literally stack holes on top of each other at the range. I would have no problem taking deer, bear and even elk at normal hunting ranges. It also makes a perfect light weight varmint rifle.

Point being, the extra 50-100fps just isn’t worth the cost and time needed to go AI with hammers. Just my thoughts and definitely not trying to sway anyone that likes to build and play….I know sometimes that is half the fun.

Same mindset applied to the 338 lapua vs the 338 edge…..thus why I have and love the lapua.

 

[gltaylor]

I think from my reading and personal experience, the max improvement you can expect from an ackley imp chamber is about 10%. Most cartridges offer less - especially the bigger the case capacity.
I absolutely hate case trimming - which is the main reason I went to several AI cartridges.

 

[Edd]

I stole this a long time ago. I don't remember where.

 

[Reemtyj]

No 338-06AI on there, probably, because it is old enough that the 338-06 Asquare wasn’t factory then

 

[BFD]

I stole this a long time ago. I don't remember where.

View attachment 3921

That chart is pretty much correct for the two AI I tried!

 

[LightTheTower]

Then there’s the 280 REM conundrum I’m in. Good brass is impossible to find, but high quality 280 AI brass is quite easily found…

 

[Farleg]

I think from my reading and personal experience, the max improvement you can expect from an ackley imp chamber is about 10%. Most cartridges offer less - especially the bigger the case capacity.
I absolutely hate case trimming - which is the main reason I went to several AI cartridges.

Gday gl I’ll give that to the ai & totally agree with trimming as that sux

Hmmm another reason to push hot loads yep no need to trim lol as 2 shots & in bin
Cheers

 

[CBH Australia]

Interesting write up. Glad your ok..

Interesting story and I do wonder if sizing down.308 to .284 if that also affects case neck thickness. Squeezing it down would not draw it out but that's theoretical. A wise man told me to use proper headstamped brass.

I have a .308 target rifle with a match chamber and I have to small base size any fired case to reload. I have rebarrelled sporters that can get a tight group on a cold barrel so is a match chamber necessary?

I have a 7mm-08, it's been great on pests from a 20" Tikka CTR. I was of the opinion it would be versatile in a short action. Pumped 200 factory rounds through it and many handloads on mostly pests. Foxes, Pigs, cats and some yabbie bait.

Some dot.com friends later enlightened me to the .280a.i. I had that built on a Tikka action. Readily available dies and brass at reasonable prices.

The chart showing the a.i. gains across the range is interesting. Many have said the .280 was one to benefit from the improved shoulder angle. I've also been told the .308 gain was negligible.

To see the 7x57 and 6.5x55 get a higher percentage of gain is interesting but I expect that's a gain from increasing the case volume as the wall taper is also straightened.

@Mountainman , consider the .280a.i. , it's the easy button and we'll supported with brass and factory ammo.

 

[les]

I agree with Mountainman unless you are looking at a short action.

 

[BFD]

Interesting write up. Glad your ok..

That makes two of us!

Interesting story and I do wonder if sizing down.308 to .284 if that also affects case neck thickness. Squeezing it down would not draw it out but that's theoretical. A wise man told me to use proper headstamped brass.

Lapua 308 new brass neck wall - .014". That same brass necked to 708 wall thickness - .015-.0155. That same 708 case after fireforming to 708AI wall thickness .016. This gives zero case neck chamber clearance and thus the absolute need to neck turn to .013".

I have a .308 target rifle with a match chamber and I have to small base size any fired case to reload. I have rebarrelled sporters that can get a tight group on a cold barrel so is a match chamber necessary?

I think "match" is a slightly arbitrary word. "Custom" is what I am all about. If my custom measurements of a reamer/chamber make it a "match" whatever. I don't shoot matches anyways.

I have a 7mm-08, it's been great on pests from a 20" Tikka CTR. I was of the opinion it would be versatile in a short action. Pumped 200 factory rounds through it and many handloads on mostly pests. Foxes, Pigs, cats and some yabbie bait.

Heck yeah!

Some dot.com friends later enlightened me to the .280a.i. I had that built on a Tikka action. Readily available dies and brass at reasonable prices.

The chart showing the a.i. gains across the range is interesting. Many have said the .280 was one to benefit from the improved shoulder angle. I've also been told the .308 gain was negligible.

To see the 7x57 and 6.5x55 get a higher percentage of gain is interesting but I expect that's a gain from increasing the case volume as the wall taper is also straightened.

@Mountainman , consider the .280a.i. , it's the easy button and we'll supported with brass and factory ammo.

This is very intriguing! I will finish my learning journey on the 708AI and short action. I have a 7mm barrel blank arriving at an unknown time in the future. I could go 280AI with that possibly...

 

[CBH Australia]

Does Joe like your 7mm-08a.i?

I'm assembling parts for a .260 Rem build but if I get to burn that out I might then need a 7-08a.i.

 

[joe16]