Can a 25-284 die be used to neck down regular 284 cases or is there a special die one needs to use?

It all depends on the brass I have heard some 284 brass varies from lot to lot. I have some brand new winchester brass that I can neck down using a 25-284 sizing die. I usually know if I can do this if I don't collapse the cases. If I start collapsing the neck then I run it throug a 6.5-284 sizing die first then I run it through the 25-284 sizing die. I also had a bunch of once fired 6mm-284 cases when I necked them up to 25-284 I was splitting the necks. So the answer to your question is yes.
Also if you need some data I have some.

John

Yes, I can use all the data you have

These were shot out of a my 25-284 built on 98 mauser with a 25 inch Montana rifleman stainless steel barrel. None of the loads showed any signs of pressure. I do not know the velocity since I do not own a chronograph.

55.0gr IMR 4831 W/100gr Sierra BTSP didn't group with a darn.
Second Load
53.0gr RL 19 W/100gr Sierra BTSP shot real well, less than 1 inch groups showed a lot of potential.
The third load.
60gr H-1000 W/ 100gr Sierra BTSP same as above lots of potential.
The Fourth load.
55.5gr RL-25 W/90gr Sierra BTSP Excellent load less then 1/2 inch groups
The other loads I tried didn't do so good.
52.0gr H-4350 W/100gr Sierra BTSP shot horrible
55.0gr AA 4350 W/90gr Sierra BTSP shot horrible
54.0gr IMR4350 W/100gr Sierra FBSP shot horrible
44.0gr Varget W/100gr Sierra BTSP and FBSP

The following loads were developed by another AR member and he was kind enough to send them to me I have not tried them yet. Like all data start 10% below.
85 gr. - Nosler Ballistic Tip, 56.0 grs. H4831sc, WLR ~ 3387 fps, .76" @ 100 yards 3 shot

100 gr. - Barnes XLC, 59.0 grs. H1000, WLR ~ 3276 fps
- Barnes XLC, 61.0 grs. H1000, WLR ~ 3396 fps, 1.12" @ 100 yards 3 shot
- Hornady SP, 58.0 grs. H4831sc, WLR ~ 3510 fps, .57" @ 100 yards 3 shot
- Sierra BTSP, 58.0 grs. H4831sc, WLR ~ 3449 fps
- Hornady SP, 54.0 grs. H4831sc, WLR ~ 3342 fps

110 gr. - Nosler Accubond @ 2.83", 53.0 grs. H4831sc, F210M ~ 3177 fps, .86" 3 shot 100y *3 groups*

117 gr. - Hornady BTSP, 52.0 grs. H4831sc, WLR ~ 3087 fps, 1.44" @ 100 yards 3 shot

John

You can try it, but remember Hornady makes 6.5-284 brass and I believe Hornady anneals all their necks.

http://www.cliffsgunsmithing.com/Catalog_Horn_R_Brass.htm

-Spencer

I just use my regular 25-284 sizing die to neck down the brass.

But...be aware...you may need to inside neck ream.

Fire a bullet from your rifle, and then see if the fired (empty) brass will allow you slip a .257 bullet in the neck. If the bullet won't go in easily, you need to inside neck ream.

You will really love this cartridge. Try the 100 grain Barnes TSX.

Garrett