Custom chamber vs cip/saami

[Mtl74]

Hi,

Whats the benefit of making tighter chamber than Cip or Saami? I understand that sizing less brass with custom sizing die vs FL sizing die ( Cip/Saami) extends the case lifespan when there is less material moving. But the other thing.. Is it the throat issue, that when made too long (Cip or Saami) one might have problems with some bullet designs when seating the bullet close to lands due to the case neck lenght vs saami or cip throat? Understand concentricity is better in a tight chamber also? Planning to build a custom 308Win for F-Class and thinking which way to go. Thanks.


Br. M

 

[jackieschmidt]

A SAAMI chamber is designed to chamber and safely fire any commercial round available, Dimensions must reflect this. That includes long throats. If you buy a Remington 700 in 308, it will be able to shoot any weight bullet commercially available.
Custom chambers simply take into account the fact that the shooter can hand taylor his ammunition to fit the needs of what ever shooting endeavor he is involved in. The throat is a huge consideration. In Short Range Score, where we shoot short 112/118 grn bullets, we use no throat or freebore at all. In a short range group rifle, you might have a .050 throat to position the bullet in the neck exactly where you want it.
in a chamber that uses a higher BC bullet that is usually much longer, you need a longer throat to accomplish the same thing. Place the shank of the bullet in the case neck in an optimum position, usually where the shank avoids the neck shoulder junction. This also optimizes the overall capacity of the case.

Other chamber dimensions, such as the actual size, can be optimized in a custom chamber to reduce case expansion, increasing case life by avoiding overworking the brass. This also helps keeping things aligned when you chamber a round. Custom dies are available to properly size the case to fit that chamber.

Then there is the neck. You can use a chamer that requires you to turn the necks to a specific wall thickness to optimize consistency in neck tension and to insure better bullet alignment when the round is chambered. Many shooters participate in Disciplines where the general consensus is that this is not required. Others believe it is mandatory.

It all comes down to how many variables you wish to remove from the “accuracy equation”.

Of course, there are multitudes of other factors involved. Always keep in mind that while there are quite a few things that will cause a bullet to take the exact same path as the one before, there are countless things that will make it go astray.
,

 

[Mtl74]

Thank you for the reply jackieschmidt, this for very helpfull.

 

[Alex Wheeler]

Be careful tightening up case dimensions unless your experienced in designing dies. You can get yourself into a spot that you cant make a die that will work. If unsure, dont tighten up case body specs.

 

[carlsbad]

Be careful tightening up case dimensions unless your experienced in designing dies. You can get yourself into a spot that you cant make a die that will work. If unsure, dont tighten up case body specs.

Alex, I've seen you post about poorly fitting dies. One way to fix that is to order the resizing reamer at the same time you order your chamber reamer. Send the reamer maker a sample of the brass you intend to use and have custom reamers cut for the brass and the die. of course you may not be able to change brass after that.

--Jerry

 

[Alex Wheeler]

Alex, I've seen you post about poorly fitting dies. One way to fix that is to order the resizing reamer at the same time you order your chamber reamer. Send the reamer maker a sample of the brass you intend to use and have custom reamers cut for the brass and the die. of course you may not be able to change brass after that.

--Jerry

Jerry, Im talking about the design of the chamber reamer and the resize reamer. You cant just tighten things up and always get a resize reamer to work. The issue is that when you reduce the chamber size in some cases you can't make the resize reamer small enough to work the brass and not get clickers. You have to know how much you have to squeeze a certain case head to get it to move enough to work. Most reamer makers will use the same numbers for their resize reamers no matter what case head your working with and that is a big problem. All the different case heads require different resize reamer specs and when you tighten up a chamber to the point that the resize reamer has to be smaller than the solid web you will start to have problems. Virgin case head, chamber diameter, and resize die all have to work together.

 

[clowdis]

I've gotta agree with Alex here. I've tried using tighter dimensions in the case body area and got nothing but sticking brass when the loads approached "hot". So after that experience I only alter the neck diameter, shoulder angle and throat dimensions on custom reamers. I can't see any accuracy gains by altering the case body section of the chamber. I think accuracy is in the neck, shoulder, and throat areas of a chamber.

 

[Alex Wheeler]

I am not advocating for slop. I should explain.
You can do things with one case you can not do with another. A 6br case has a relatively small solid case head. Not a whole lot of solid brass is in the chamber and the case walls are thin. You can tighten up on this case a bit and make a die that will size it enough to work. Step up to magnum or lapuas and depending on the make of the brass, you can tighten up the chamber and do to the thickness of the brass and how much solid web is in the chamber you can easily get to a place where no die will work. So , like I said either you have experience with the case or your in the mood to gain experience