I have a sako 75 Varmint on layaway and have ordered a Mcmillan stock for it. I am wondering if it could be re-chambered to 6mm AI......I have som questions.

1. does the sako 75 .243 action have enough length to seat pills out to get best powder capacity and still feed through the Magazine (it seems a little bigger than a rem 700 .308 length action)

2.When reloading this round do you fireform first and then work up using 6mm rem mag data.

3. This site has data for the 6mm AI but only for 100 gr pills it gets 3278 with these, what speeds could be acheived with 75, 80, 85 & 90 gr pills ??

cheers PC.

A 6mmAI should clean up the chamber.

Yep fireform and work up

I have no clue as to the magazine length

quote:

Originally posted by ramrod340:
A 6mmAI should clean up the chamber.

Yep fireform and work up

I have no clue as to the magazine length

Are real world velocity increases signifiacnt over the .243 Winchester ??

quote:

Are real world velocity increases signifiacnt over the .243 Winchester

Guess the question is what do you consider significant?
The basic 6mm has a couple grs more capacity than the 243. In factory the key issue is the the 6mm is loaded to 65,000 vs 60,000 for the 243. Handloaded to the same pressure there is basically no difference. I have converted a 6mm to AI capacity gain was around 7.7%, velocity increase as I measured it was right at 2% or about 66FPS.

Looking at my Nosler manual. Max velocity for the the 243 and 6mm are pretty much the same. My real world testing added about 66 fps from a 7.7% capacity increase. Looking at the 240Wby which has 20% more capacity than the 243 or 6mm the velocity is only 100+/-fps higher. Pretty much the same for the 6-284 and other 06 based wildcats.

So in "MY OPINION" loaded at equal pressure the increase to AI is not significant nor is the increase to the 240Wby. Some will argue 100fps is worth it. Other I'm sure will claim much larger gains. What I found was the AI case doesn't an excellent job of hiding pressure signs so many load in excess of 70,000. Huge gains I feel are pressure gains not capacity. Many have no issue with loading to 70,000 saying the gun and brass will tell you when to quit. I did myself for years. I just got tired wondering is the load I worked up at 70deg would blow the primer at 90deg.

Now that I'm sure I have confused you. My answer is stay with the 243. If you really want a 6mmAI go for it. But expect less than 100fps gains. Only you can decide if it is worth the rechamber, new brass, dies etc. And as I have said before this comes from a shooter with more AI and full widlcats than factory.

Paul's comments pretty well match my experience. I have virtually identical .243 and .244 HB varminters. Top velocities with 70 grain Nosler Ballistic Tips (great accuracy and terminal performance) with the .243 is just a shade under 3600 fps, and with the .244 is just a shade over 3600 fps (both 24" barrels), but both are withing 50 fps of one another. I would expect the AI version of the .244/6mm Rem to add no more than another 50 fps at similar pressures.

These are both on Sako L579 actions. With varmint weight bullets, you have plenty of room for the 57mm case of the .244/6mm. If you move up into "deer" weight bullets, then you have to seat the .244 bullets pretty deep. Assuming this is a dedicated varmint gun, and assuming the Model 75 magazine is of similar length to the L579, then the magazine length on the Sako is adequate.

However, there is an other possibility: A friend has an L579 that has been rechambed to .243 AI. He really likes it. Velocities will pretty well match the standard 6mm, but you have more flexibility in the magazine and generally have an easier time finding more pletiful brass. Just a thought.

.243 AI.....................just of running the reamer up the Barrel ?? and then fireforming .243 winnie brass ??

POssibly it could give me an extra 50-100 fps ??

may be thats the easier option.

Thanks Guys.

quote:

.243 AI.....................just of running the reamer up the Barrel ?? and then fireforming .243 winnie brass ??

POssibly it could give me an extra 50-100 fps

Sorry not that easy. To be done correctly you need to set the barrel back one turn then chamber for the AI. While some do it the way you are talking about they then have a headspace issue when they try to use factory 243s in their chamber. Factory ammo should be a crush fit in an AI chamber.

I've tried the AI version of the 243 and 308. Per my data you gain about 1.5% capacity from a 243AI. Normal shoulder on an AI is .454 the 243 is already there so you change the shoulder from 20 to 40 but don't reduce the taper. I saw basically no gain when I built a 243AI. In my case I just burned a little more powder for equal velocity. This conversion was done in the same barrel. 50fps would be on the high side of my quess.

Load your 243 to 65,000 like the 6mm and use the $$ you save for more shooting.

Again, Paul speaks from experience.

I would differ with him only in the necessity to set the barrel back. Since you will be handloading, anyway, you have the choice of making your chamber as "long" as you wish. The only disadvantage will be that factory loads will have excessive headspace and should not be used.

If I were going with a .243 "AI", my suggestion would be to make the chamber on the long side, allowing you to skip the troublesome removal and refacing of the barrel. Then I would use exclusively .308 brass, resized to be what some people refer to as a "crush fit" in your improved chamber. The .308 brass will have a thicker neck, but this will allow you (if necessary) to thin the neck walls for a custom fit in your chamber.

Should someone insert and fire a standard .243, it will have "undesireable" headspace, but the worst result will be a fired case that is stretched enough that it should be discarded. Please note: If using factory .243 ammunition is important to you, then by all means do as Paul suggests and set the barrel back in order to achieve minimum headspace.

At any rate, I would not expect more than 50 fps out of this effort. But hey, go for it if it interests you. If we really only built the rifle calibers we need, things would have stopped with the .22LR and the .30-06.

quote:

If I were going with a .243 "AI", my suggestion would be to make the chamber on the long side, allowing you to skip the troublesome removal and refacing of the barrel. Then I would use exclusively .308 brass, resized to be what some people refer to as a "crush fit" in your improved chamber. The .308 brass will have a thicker neck, but this will allow you (if necessary) to thin the neck walls for a custom fit in your chamber.

There is nothing at all wrong with this method if you plan to treat the rifle as a full wildcat. I have done it myself. But I will only shoot reloads sized for the chamber and won't sell the rifle marked as AI. Only real issue I have with it is someone later firing factory ammo. While it might not result in a major problem it isn't exactly safe either.

Like I said if it were mine I would load to 6mm pressures and shoot it a lot. If I wanted an AI or wildcat and was willing to spend the $$ I would opt for the 6mm version. I would not do a 243AI.

PC,
I have a strong suspicion that Shane Clancy runs a 243AI as his varmint rifle ... or used to. He has the reamers and would be worth a chat.

But here's another option. 6mm/284 ... I've got the 26" chambered/threaded stainless barrel, ordered two piece Redding dies plus a Hornady Universal 6mm neck die ... just waiting to hear back on a possible donor action. In theory ... 3500fps with 80gr from a 26" barrel. My first one will be a "bitza" and will be put together with a "no-turn" neck, then if I like it I'll get another done. Added bonus is the excellent Lapua 6.5/284 brass available.
Cheers...
Con

Con the 6mm/28sounds great but I want a repetaer and for a repeater in that cal I think I would need a long action ??

I dont think I will bother with the .243 ai

quote:

Con the 6mm/28sounds great but I want a repetaer and for a repeater in that cal I think I would need a long action ??

The 6mm-284 should fit in an action that will handle a 6mm. While I feel it is at it's best in a long action it works in a short. The bullet just eats up some space. Real world probably 100fps over a 243. Same level of capacity as the 6mm-06 or the 240Wby

Well maybe thats an option.......brass is available, I get 100 fps second increase and I can have a repeater..........get the chamber cust so I can be on the lands with the bullet seated so I can still use the mag.

PC,
At one stage I was going to have my barrel fitted to a long-actioned M670 whilst another used a short-actioned Tikka T3 so we could compare the difference. Now ... I'll just place mine on a short-action and it will definately be a repeater. When the barrel's fried it'll morph into a 375/284 or another 6mm/284 if I decide I really liked it. I'll basically be shooting 75-85gr projectiles with maybe a few 100gr thrown in as I have access to a fallow deer property where you can really stretch a rifle out whilst the deer are feeding in the paddocks.
I've Pm'd Shane Clancy quite a bit on the 284 wildcats as I'm kind of interested and have already collected 30/284 and 375/284 dies with 6mm/284 coming. He's pretty confident of getting them all to feed and has built them all for various customers from 22/284 upwards ... he'd be worth a chat for your project.
Cheers...
Con