338 lapua AI or 338 lapua imp

Mutt-roc

Mutt-roc

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i was at the range today and a guy was shooting a 338 lapua imp. the difference between the imp and the AI is the imp has a 40 deg sholder and the AI has a 38 deg shoulder. now what r the pros and cons of the 40 deg over the 38 deg. any info will help thanx




your friendly neiborhood MUTT
 
The BIG difference occurs only when shooting on Thursday afternoons AFTER a full moon on EVEN months.

Now if there is a match during a LEAP YEAR, you may want to consider a 38.5deg shoulder. That pretty much ensures success even if your RO is a Leprechaun.

Do keep those shoulders clean because a 40deg case will get confused in a fouled chamber and not know if it should or should not behave like 38deg case.

You got your work ahead of you trying to make the right choice. :)

Jerry

PS Smart A$$ comments aside, choose the shoulder angle that you can get Redding body dies for. Pretty sure they are always 40deg but check with a tech and get a drawing if possible. A shallower angled case will have a bulge when the wrong die is used. Not the best for brass life as you likely have to oversize to chamber.
 
I would gladly give up any slight advantage the improved cases have over the advantage the original shoulder has when it comes to smoother feeding.
bigbull
 
I'm running a 40degree shouldered 338LM and it feeds just fine. I think with the sharper shoulder then the standard it definitely helps brass life by the sheer lack of brass growth. As far as the improving the case or not, if you're a hand loader, 111.5gr of powder vs around 100gr is substantial. My bullets are supersonic out to 2700yds.
Mutt, it largely depends on how much you want to spend on a new reamer if your local gunsmith doesn't have it. If he has something that is very close and it works, go with it. I've come to realize that in the reamer world, some guys give their reamers a name after changing dimensions ever so slightly hoping that someone will imagine up a difference and make them famous.

THat's my thought.
 
i am not planning on buying one any time soon i just wanted to know if there was any big difference between them. the gun i saw this on was also made out of a rem model 700 action.
 
Max Owner said:
So there is absolutly no benefit of a Lapua magum over the Ackley Improved?
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If the difference has never been pointed out to you, you've never looked. It is phyically impossible for a standard lapua to stay supersonic out to 2700yds. The Cheytac boasts this figure. 99% of the Lapuas out there are driving 250gr bullets and with the twist rate, that's fine for 1000yd work but past that, they drop off drasticly. The other 1% being customs are driving a 300gr bullet just shy of 3100fps with a muzzle energy of 6000ft/lbs. If you have a ballistic program, use these numbers and prepare to be astounded. Use an elevation of 3000ft.
It's about to get even better with the release of the Lapua 300gr bullet with an even higher BC of .810 over the SMK with a BC of .763.
 
Mutt, I hope the gun you saw was a 338 Edge which is a 300RUM blown out to 338. There isn't a lot gained by straight walling this cartridge and putting a sharper shoulder on it aside from longer brass life but RUM brass is cheap in comparison to Lapua brass.

The straightening (improving) of the case wall and changing the shoulder angle from the 20 plus degrees of factory 338 Lapua to 40 degrees adds about 10 % case capacity, which is clearly a significant improvement.

If someone has bubba'd the bolt face of a Rem to take a Lapua, the fact that he would have cut most of the bolt face away and there isn't much material left after the fact would scare me into shooting as far away from that guy as possible. Remington has taken alot of time to finally come up with their own 338 Lapua I suspect trying to deal with the bolt face issue, at least they are in a good position to know what and where the stress points are in the bolt and can temper it properly, not to mention they carry HUGE liability insurance. You never hear about the guns that blow out the side of the bolt face unless someone else witnesses it.
 
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My bullets are supersonic out to 2700yds.
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What bullet are you using to get 2700 yds? I ran the numbers for a 300 grain SMK at 3100 through a couple programs and the max I could get out of it was about 2300 under normal atmospheric conditions. Just curious.

-John
 
I was talking to the man in Calgary about the 338 Lapua in a 700. He explained how they made it work. I still think I would rather go with a different action for a Lapua myself. Saw a nice prototype that would work quite well.
 
I used Exbal from Nightforce and these are the numbers I ran.
2700yds 110.50MOA w/400yd zero 1008fps 677ft.lb of energy 4.9395 time of flight

I used 3400ft elevation; 3078fps
I'm able to seat them slightly longer then I originally shot them and I'll crono them again.
 
John, my ignorance amuses me. I guess my gun is only supersonic to 2350yds but that's only 81MOA over my 400yd zero. I'd still like to find out if these bullets start to tumble or what after they drop below supersonic. I'll have to do more testing.
 
2350 yards is still a serious piece of real estate. What really impressed me was the trajectory difference between the standard Lapua Mag and the Improved. something like 7 MOA at 1500 and 9 MOA at 1 mile. Are you single loading those rounds or feeding them though a magazine?

-John

P.S. From what I hear the Sierra 300's seam to handle the transition to subsonic very well. I haven't had an opportunity to test that, just what I've heard from some guys south of the border.
 
10% case capacity increase will net you roughly a 2.5% velocity increase, all else being equal (which it rarely is, but still). The improved shoulder and less taper gets you that and slower brass growth. Whether or not it's worth it is up to you. The 338 Edge is the 300 RUM (not the 338 Rum, which has a shorter case), blown out and necked up to 338 cal. FWIW - dan
 
Guys, it all depends who's reamer you use. Of the two Edge Reamers that I know are being used, one is a standard that simply blows the cartridge out to 338 and the other sharpens the shoulder. The 300RUM casing is actually longer then a Lapua casing but since it's narrower and literally straight walled already, there isn't much casing expansion. The 338Lapua on the other hand has huge growth in an AI form.
 
Johnny5 said:
What bullet are you using to get 2700 yds? I ran the numbers for a 300 grain SMK at 3100 through a couple programs and the max I could get out of it was about 2300 under normal atmospheric conditions. Just curious.

-John
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I didn't hear him make the claim he's doing anything at that range. Most individuals shooting .338 LM are suggesting around 1,600-1,800 yds. as maximum effective range for the cartridge.

Where some of you are comming up with 2,700 yds. is beyond me. What difference does it make if it's supersonic if it isn't accurate?
 
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