7mm WSM and .270 WSM

I have a specimen of each of these rounds. Apart from the shoulder on the 7mm being slightly further forwards there is almost no visible difference. Is the shoulder on the 7mm slightly further forward to prevent the 7mm fitting in a .270 WSM chamber? I know that the .270 has a .277" bullet, and the 7mm has a .284" bullet, but why produce two different cartridges that are very nearly identical. Does one have any ballistic advantage over the other? If there is any, I would think it would be small.

Thanks in advance for any info.

One of the major reason anything is produced in the US is not need it is what you want. The .270 is and continues to be one of the more popular rifle rounds so that may be the reason for the .270 WSM and about the only straight metric rifle measurement to gain popularity was the 7mm mag. This is just an opinion that and $1.00 will get you a cup of coffee. vic

I agree with Vic…two popular cal’s Just a mater of preferance . I have a 300 WSM rifle and love it! Would not mind trying the 270WSM, or the 325 WSM.


While I think the .284 bore is more popular than the .277. As there are only 3, .277 caliber cartridges out there, the .270 WCF, 270 Weatherby Mag, and the 270 WSM. While in the .284 caliber there is the 7mm Mauser, 280 Remington, 7mm Remington Mag, 7mm-08 Rem., 7mm WSM, 7mm Rem. Ultra Mag, 7mm Rem. Short Action Ultra Mag, 7mm STW, 7mm Weatherby, not to mention the 7-30 Waters and the 284 Winchester. But, like Vic said that and a $1.00… But yes the reason for the shoulder difference is to make sure no one chambers a 7mm WSM cartridge in a 270 WSM chamber. This is not my opinion, I read this in an article back when the 7mm WSM was introduced. I wish I could remember which magazine and issue it was. But I think my old age is catching up with me.

It’s just my own ignorant opinion but I think it would be difficult to chamber a 7mm WSM in a 270 WSM rifle even if the shoulders were the same. The difference in neck diameter would preclude that. Unless you had a very sloppy 270 WSM chamber.

OTOH, a 270 WSM can be chambered in a 7mm WSM chamber and the shoulder difference could create an excessive headspace situation. Maybe not as dangerous as the first situation, but dangerous none-the-less.

You’ll find all sorts of examples of being able to chamber the wrong cartridge and some of them are downright dangerous. That’s why the manufacturers have lawyers and liability insurance.



Didn’t mean to offend. Sorry.


I can’t see where you offended anyone. Was it something that I said??


Just thought maybe I had.

I did some digging and found the magazine article. It is in the Shooting Times Sept. 2002 issue. The story is by Layne Simpson. I will quote from the article. "There was some delay in the production of 7mm WSM rifles and ammunition because Winchester decided to pull in the oars along about mid-stream in order to make a dimensional change to the case. The first version was simply the .300 WSM case necked down for bullets of .284-inch diameter. Before rifles and cartridges got much farther than the drawing board, however, Winchester technicians realized that it might be possible for a shooter with bulging muscles to force a 7mm WSM cartridge manufactured on the minimum side of the SAAMI dimensional tolerance range into a rifle with a chamber reamed on the maximum side for the .270 WSM. Even worse, that same fellow might even decide to pull the trigger after he had forced the wrong cartridge into the rifle. To make certain this never happens, Winchester went back to work, and as a result the shoulder of the 7mm WSM case was mover forward just enough to prevent it from being chambered in a rifle in .270 WSM."
To be honest I can’t believe I found the article.

You could say very much the same for the .270 Weatherby Mag and the 7mm Weatherby Mag. Check out the dimensions of those two little stable mates.
I wonder how many times those have got crossed over. "The clerk in the store assured me they were the same"
You could quite probably fire the smaller bullet in the larger rifle and not even realise except for poor accuracy - but don’t try it to see!

If you are talking about the risk of chambering the wrong round in a rifle the belted magnums must rate absolute highest on the danger list.

With a rimless cartridge the case headspaces on the shoulder so even a small difference in length is enough to prevent you closing the bolt or the round dropping in too far to be fired. So although there is always a risk, much of the potential for error does not actually reach the final consequence ie a blown up rifle and an injured shooter.

Most of the belted cases share a common set of base dimensions that can be traced back to the .375 H&H Mag. and they all chamber on that identical belted base.
Some later cartridges, like the Weatherbys, changed the dimensions slightly but not enough to make them safe.
A wrong cartridge will therefore chamber like it was meant to be there and fire.

Cartridges in this catagory include

.375 H&H Mag, and its decendants

.244 H&H Mag
.257 Weatherby Mag
.275 H&H Mag
6.5mm Rem Mag
.264 Win Mag
.270 Weatherby Mag
7mm Weatherby Mag
7mm Rem Mag
.300 Win Mag
.300 H&H Mag
.308 Norma Mag
.300 Weatherby Mag
8mm Rem Mag
.338 Win Mag
.340 Weatherby Mag
.350 Rem Mag
.358 Norma Mag
.458 Win

I once knew a fellow who chambered and fired a .308 Win. in a 25-06 gun. I know he did because I have what was left of the .308 case and his safety glasses. It didn’t do much for the gun either.

The one that seems to occour most often is a .243 win in a .308 win rifle. Those being (probably) also the two most popular deer size rifle calibres in the UK at the present time.

A lot of people own both calibres although the .243 is not legal for all types of deer. The sport of long range fox shooting is growing and the .243 is one of the favoured calibres for that but I think its a bit of an overkill.

However, when it occours it does “fail safe”. The case does some spectacular fireforming but the .243 bullet just rattles its way down the barrel and out into the wide blue yonder.