Suge Knight has no doubt Diddy would have shot at him if Diddy tracked him down at a famous restaurant in L.A. ... but his story will never be heard at Diddy's trial, even though it feels pretty important to the prosecution's case.

The former Death Row Records owner tells TMZ ... his rivalry with Diddy was so bad back in the day, Diddy would have had no choice but to use a gun if their paths ever crossed in public.

That almost happened back in 2008 at Mel's Drive-In ... because Diddy's former personal assistant testified this week he went to the famous diner to get cheeseburgers for Diddy, and bumped into Suge.

David James, the ex-assistant, says he was with one of Diddy's security guards, D-Roc, when they spotted Suge at Mel's ... and when they went back to Diddy's with the food, Cassie was crying and Diddy, armed with 3 guns, got into a different vehicle with James.

James said they drove back to Mel's but Suge was gone ... and Diddy ordered him to drive around the block, but they never encountered Knight.

Suge tells us Diddy having 3 guns is a sure sign the weapons would have been used ... and he says if Diddy was on alcohol or drugs -- there's been lots of testimony about his alcohol and drug use -- particularly coke, violence would almost certainly ensue.

We asked Suge if he was packing at Mel's ... he refused to answer, but listen to the way he responds and draw your own conclusions.

Suge and Diddy's history dates back to the mid-'90s at the height of the East Coast-West Coast rap wars that eventually ended with the murders of Tupac Shakur and The Notorious B.I.G., who was shot not far from Mel's.

Prosecutors are trying to nail Diddy on a racketeering charge, and it's kind of crazy that in some ways, Suge might be essential to convincing a jury Diddy was running a criminal enterprise.

What better way to nail Diddy on a RICO charge than to illustrate the feud between Bad Boy and Death Row? And Suge talking about the beef reaching a point where guns were likely to be used sounds pretty compelling.