First off, the vast amount of experience and knowledge held by the members of this site should make you want to jump in head first with arms wide open and with both feet. I agree, it was a mistake to reference your questions simply toward me as there are MANY on here that have both 7mm RUMs and 7mm AMs built by me that can offer you a wide range of experience which is ALWAYS better then information from a single source.
The experience on this site is tremendous even if it may not me like minded all the time, you would be very wise to take all the information offered by everyone as there are thousands on LRH that can offer you information and that is vastly superior to getting it from one source.
Now to your question.
The 7mm AM is a monster 7mm. As such, its velocity potential is such that it can be very hard on certain bullets and as such, special consideration must be taken to get good results from it. The main concern is with thin jacketed bullets because at the velocity the AM can drive them to, if they are rotated to quickly, they can come apart at worst and at best, accuracy can suffer.
That is one of the main reasons that I recommend using bullets of at least 175 grains in the 7mm AM. I am not saying bullets lighter then that can not be used with great results but it raises some special considerations.
Now if you shoot the 160 gr Accubond, drive it as fast as you want, you will not tear it apart, even at 3550 fps out of a 27" barrel length. The 168 gr Berger will also have similiar velocity potential which is why it becomes tricky. On average, in my test results, with a 1-9 twist barrel, the Berger bullets as well as the Wildcat bullets and any bullet based on the J-4 jacket or similiar thickness jackets will shoot very well up to around 3300 fps, much over that and accuracy drops off dramatically and you can even see some bullet failures in flight.
This is why its designed around +175 gr bullets. THe 175 gr SMK has a relatively thick jacket and will allow the 7mm AM to rev up to its full velocity potential of around 3400-3450 fps with that bullet weight. A 200 gr Wildcat bullet will drop velocity level potential to some degree which is why it performs very well with the right barrel at around 3200-3300 fps depending on barrel length again.
I never say that the 7mm AM is THE best 7mm on the planet because that is silly. There are more situations where the 7mm AM would not be the right chambering then where the 7mm AM is the correct choice. I talk more customers out of the 7mm AM chambering then recommend it.
Generally, if someone came to me and said they wanted to shoot the 162-168 gr class VLD bullets at high velocity, I would first recommend the 7mm RUM over the AM. Why??
Well, if you take a 168 gr VLD in a 7mm RUM and load it up to the rounds top velocity potential, you will be around 3200-3300 fps depending on barrel length, right at the top end of these bullets working range and they will perform very well.
In the 7mm AM you would have to pull the throttle back to drop velocity back to the range where these specific bullets work best, sub 3300 fps. This can cause other probems, such as possible velocity spread increases and certainly increased carbon fouling.
The trick is to get a chambering that will get you the velocity you want but do so in a working pressure range that will allow these big cases to function as cleanly as possible.
So, if this is the bullet weight you want to use, I would recommend the 7mm RUM.
If you wanted to shoot a bullet such as the 160 gr Accubond or 175 gr SMK or even the 180 gr Berger, the 7mm AM is a more viable choice. With the 200 gr ULD RBBT, there really is no better choice then the 7mm AM if your looking at shear performance.
Twist rate. I have never built a 7mm AM designed to shoot the 168 gr Berger. If I were to, I would likely use a barrel with a twist rate of 1-10 or even possibly a 1-10.5 twist. The reason is because of the AMs velocity potential, you have to keep your RPMs down if you want to give the bullets the best chance to survive the launch.
For the 200 gr ULD RBBT, the 1-9 twists work very well when loaded to top velocity. I have also tested 1-8.7 twists and they work very well. Anything faster then that is really not needed and becomes harder on bullets.
With this level of performance, its a fine line but if you hit it right, special things can happen and we are figuring the 7mm AM out pretty well. I have had great results with the 180 gr Berger with slighly less then max loads but you have to stay on top of carbon fouling or it can be a problem.
Back to my original point, thanks for your kind words but you would be much wiser to ask from the entire groups experience then one person. Not only will more be willing to offer you information but you will get a wider range of informaton and again, that is always more valuable then getting it from just one source.
Just do not limit your resources that way next time.
