6.8, 6.5, or 300BLK

[jonp]

"300 blackout is joke of a cartridge"

I sure wouldn't go that far. Depending on the distance this round will do nicely for deer all while using 223 brass which is plentiful and cheap with the same magazines as any 5.56 AR. Swap tops with a 223/5.56 and you have 2 rifles in one.
I know you can form Grendel brass from 7.62x39 but I think the AR brass is cheaper and easier to get.

I have the 300BLK and the next gun is going to be a 6.5 Grendel, though.

[dh2]

I have both just because I wanted to play a bit, 6.5 and 6.8 I have no issue about hunting deer with either , the 300 BLK is not up to the distance. brass and loaded ammo is not an issue Hornady makes the 6.8 and Fed. makes the 6.8.

[TheGrimReaper]

I am a Grendel man myself. Just got the Howa Mini to play with.

[bruce drake]

I ended up recutting the chamber of one of my 300BLK uppers (I have 2) into a 30 Apache (7.62x45) which is the 223Rem case opened up to 30 caliber. It limits the bullets to 150gr or smaller (I prefer to load it with 110 and 125gr spirepoints but I can load a 150gr FN for heavier impact but I lose needed velocity) but the new chambering allows me to use the 223Rem cases without all the cutting and trimming back to the 35mm case length and the use of more medium-speed powders (I use an adjustable gas block to bleed off any excess gas pressure should it get a bit of a spike from my carbine-length pigtail gastube) I consider it my "30 Carbine on steroids".

Regarding the 3 cartridge choices of the original posting, I would take the 6.5 Grendel for a couple of reasons since this is a cast bullet forum I'll couch it as follows.
6.8 SPC - 270 caliber molds are very few. very limited in bullet options. The Case is also a one-off. You can't easily reform another case to keep your rifle firing during periods of component shortages. (you can cut down 30 Remington cases but those are rarer than 6.8SPC!)
300 BLK - 30 caliber bullets molds and designs are out the yazoo. 223 Remington donor cases are always available for resizing and case forming. - cons are that the powder choices are very limited due to the small case (35mm in length) and anything over 125gr in bullet weight won't achieve supersonic velocities. Heavy subsonic bullets are great if you are shooting through a suppressor or short range from a tree stand but anything past 50 yards and you are lobbing them in like a mortar. And to form the 300BLK requires you to cut 10mm off a standard 223Rem case and then reform and potentially turn the necks of the reformed cartridges to fit your rifle's chamber if the brass is military surplus and thicker than commercial.
6.5 Grendel - A great compromise cartridge. - bullets and cast bullet molds can go from 90-140gr in weight which allow you to go from groundhog to deer in capability with velocities nearing 2700fps with the jacketed 100gr bullets. The cartridge case is based off the 7.62x39 case and can actually be fireformed in your rifle from a necked down 6.5 (from 7.62)x39 upon initial firing. Powder selection is better with most medium speed powders being able to run in this cartridge case and the jacketed spitzer bullets have such a high ballistic coefficient that often marksmen can make hits exceeding 600 yards with them.

[Smoke4320]

"Should I go with 6.8 SPC, 6.5 Grendel or 300 BLK and why? The Grendel really appeals to me because one day I'd really like to start playing with long range shooting"

Based on this statement its 6.5 Grendel all the way .. I have a Blackout and a Grendel .
The blackout is a fun gun to shoot killed several deer and a pig with one . its NEVER let me down on a 150 and down shot (bullet placement and bullet construction for the job is key).. its even more fun with a suppressor and subsonic .. but a long or even medium range gun it is NOT
on the other hand the Grendel will kill a deer at yardages most people should not be shooting them at
my longest deer kill was 437 lasered yards .. off a bench like steady rest .. 20 oz co2 bottles are no match at 500 yds (again with a good rest)
with the correct load and equipment 1000 yds target shooting is no issue