I have both that I bought in the late 80's. They used to shoot really well MOST of the time. I would often get 4 nice shots and a flyer.  Many of the groups with hirtenberger ammo were incredible with the 91. Match ammo was exceptional with many under 1 1/2 inches. I got great groups at 200 and 300 also. The 93 would often shoot under 1 inch with good ammo and certain fmj ammo. It seems like the accuracy had started suffering in both before I quit most of my shooting of rifles in mid 90's. I bought two 223 bushmasters recently with one being a varminter. I resumed my shooting and was getting great groups with the varminter and really good ones with the 16 inch. I brought out the H&K's this week and neither one would shoot a decent group. The scopes were tight etc.  I finally got 3 nice groups with the 93 as I was quitting at around 3/4 inch for 5 shots. The next group with different ammo was all over the place.
Any ideas what the problem with them may be?

no real help from me, except I am looking for a 2nd HK93....so if you want to save yourself some grief.....shoot me a PM :)

Save YOURSELF some Grief. They are going for over $3000+ around here. Especially with the collapsible stock, bipod, magazine connectors and ARMS mount.
JR

Make sure that you give it a good cleaning.  My 91 was kind of so-so until about 500 rnds through the pipe, but I had to clean it often.  After that, the need for cleaing diminished and group sizes have stayed around 1" with some Yugo ball I bought in the 80's.  It likes a consistant hold.  Sloppy on the hold gives me sloppy grouping.

Could be part of the problem. I was letting them rest on sandbags with little holding like I do my AR's
JR

My understanding on the HK-93 is that the barrel twist was intended
for a lighter bullet than what is used in most 5.56 today.

I think  a 55 grain originally ,and a 75 ? grain today.

If I remember correctly ,the original 93s had a 1 in 12 twist,
and later models a 1 in 9.
Some special ones were made in 1 in 7 .

Changing the bullet weight for the older designs could severely
effect your group size.

I'm not a 5.56 guy ,so I would ask someone more knowledgeable,
perhaps someone in the AR forums would know more about
barrel twist versus bullet weight for 5.56.

All I know about it ,is that someone I shot with had his groups more
than double in size when he used the newer heavier ammo in his 93,
and he said something about this.

I'm all about .308 .
1/12 twist ,145 grain to 175 grain.
I'm not real concerned with what velocity my round will topple at .
If I hit em  ,they're worm food.  

Having the same problem with the 91 which is 308 also. The ammo I shot in the 93 was mostly 52 to 55 grain.
JR