Jerry Teo and others have given some good advice.
AS an addition here, your 6.5 Gibbs was probably made on the 06 case length.
The 270 case is about .050" longer then the standard 06 and MUST be shortened if you plan on using that brass.
The 270 case is the easiest to bring down to the 6.5 diameter. Just take the factory measurements of the 06 case and trim the 270 brass to .010" less then that of the 06 and you can use it in the 6.5 Gibbs chamber.
To fireform the 6.5 Gibbs-- seat your bullets so they are into the lands and the bolt has a snug fit when you chamber the round. This is keeping the base of the case back against the bolt face when fired. The case will form to the chamber without any case loss in the process.
I used lighter bullets and standard 6.5/06 powder suggestions to do the fireforming.
Hopefully there is an 8 twist barrel on that rifle so you can use the 140 gr to 155 gr bullets in your Gibbs.
What is the barrel length on your Gibbs?
I was duplicationg a 264 Mag with my 6.5 Gibbs with a 30" barrel.
Once again, Thanks!
You know, I am not sure on the barrel length. I will check it out and let you know. You know fellas, I am still gushing at the fact that I found someone who actualy knows about this rifle. Everyone I talked to around here just gave me blank looks.
I've got some new .270 brass, a case trimmer, powder, cream of wheat . . . yep, I'm ready to fireform. I'll let you know how it goes.
Welcome to the joys of wildcatting and shooting in general. I have been shooting a 6.5 ackley for years which is very similar to the gibbs. you have already gotten some really good feedback! 25-06 brass also works very well. in fact , fired 25-06 brass will accept a .264 bullet without even resizing the neck. This will work fine provided the fired brass will fit in your chamber? otherwise, use the advice you have already been given. another trick is to seat the bullet out to touch the lands which will help maintain proper headspacing. when i use .270 brass, i like to set the neck sizer so it leaves a bump at the base of the neck (just enough so that the bolt closes with a "little" difficulty. it takes a little trial and error, but this will also help maintain proper headspacing. this is very important! if headspacing isn't maintained during fireforming, you will soon separate most of your cases at the web when firing full loads. good luck and feel free to let me know how you do........rich
Have shot all the Gibbs for many years. I have had better luck using 30-06 brass and necking down in two steps. I get less split necks and it gives a very nice false shoulder to headspace on. Like the first guy said neck it down slowly until the bolt closes snug. That is your headspace and it will not drive the case forward upon firing causing a bad situation. You are better to use largr caliber brass than smaller because the brass expands so much into the neck of your parent brass. The Gibbs neck is very short. It will give velocities near the 264 winchester which is a plus using cheap '06 brass. I have a long range heavy rifle in this cartridge that is extremely accurate. Also the same rifle in 6mm Gibbs.
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