Another excellent post from the Ruger forum:
The Mini-30 was introduced in 1987. For the first couple years, the bore was a true .308 bore. In early 1989, Ruger decided to make the bore the industry standard .311".
A few with .308" bores slipped out 1989-1990, I presume that Ruger wanted to use up the last of the .308" barrels.
One of my 189s was made early on in the production, and it has the larger bore. I have measured all of mine at .310.5"
The 189 series (first 3 numbers of the SN) was the longest running, being made from 1987 to 2000.
The majority of used Mini-30s that I have seen have been 189 series.
Ruger Mini Thirty Serial Number History
I have had 4 189s, sold one, gave one to my daughter, and now have, and will always have two.
Compared to current production, the 189s have zero QC issues, have better workmanship, none of the rough machine marks, burrs and prominent mold seams that most of the newer ones have.
All of the Mini-30s ever made used a .625" barrel, compared to the skinny "pencil" barrel (.560" diameter) older Mini-14s. So older Mini-30s don't have the accuracy issues and vertical climbing of the groups as the barrel heats up like the pencil barrel Mini-14.
The exception to all Mini-30s having a .625" barrel are the Mini-30 Tactical, introduced in 2010. It has a .750 diameter barrel. (all of these diameters are measured forward of the gas block).
Mini-30 firing pin length has been all over the map. That's why some guys say they have shot nothing but Russian steel case for 30 years and have never had a misfire. Others have trouble getting Commie Berdan primed ammo to ignite.
When you get a new Mini, rather than guess at your firing pin length, just get a fifing pin protrusion tool from Brownell's. It will set you back $40, but then you'll know exactly where you stand on firing pin length.
BROWNELLS CENTERFIRE/RIMFIRE FIRING PIN PROTRUSION GAUGE | Brownells
Correct firing pin protrusion is critical to the entire ignition sequence. Elliptical face on the recording plunger allows accurate measuring of both rimfire ...
If you measure your pin and it is less than .040", and you plan on shooting steel case ammo, buy an oversized firing pin from firingpins.com. I'd fit it for .042" minimum. You can go a little longer, but start to risk piercing the primers of U.S. Boxer ammo, if you on occasion shoot that.
Don't listen to Youtubers and others that tell you to install a heavier hammer spring, or grind some off of the back of your bolt.
It is not the hardness of Berdan primed Russian ammo that is the problem, it is that they are seated deeper in the case. Unless you get lucky and buy a Mini with long pin protrusion, your Minis factory pin will have trouble reaching them.
Something else to check is to see of your Mini has a raised rim around the firing pin hole on the bolt face.
That ridge will have to be removed, as it effectively reduces your FP protrusion.
Another thing to consider is that not all Russian ammo is the same. Tula has the deepest set primers.
In firing a Mini I had with a factory FP, around 15 to 20 rounds out of 100 of Tula failed to go off the first time.
With Red Army Standard or Golden Tiger (same ammo, different packaging), 1 or 2 out of 100 wouldn't go off.
With Silver, Brown or Golden Bear, I've never had a round fail to go off.
I would avoid Tula unless it was all you can get. Tula is lower velocity, dirtier, less accurate, and harder for the Mini to set off than any ammo I've tried.
I've done a lot of testing the last 7 years out of my two carbines, and Silver Bear is almost as consistent and accurate as the handloads I use with Hornady and Nosler bullets.
Silver Bear has a zinc coating over the steel case, Golden Bear has a brass coating over the steel.
Both of these rounds are not much more expensive than bottom of the barrel Tula, about $1-$1.50 a box more.
For just a few dollars more a box ($10-$11) you can get real brass cases and Boxer primers so you can reload the brass. Fiocchi and Geco are two inexpensive brass cased brands that are accurate and clean burning, but I have found velocities of Fiocchi and Geco to be on the low side.
The second half of the Mini-30 580 series got a few improvements along with the later 580s in the Mini-14.
Better winged rear sight on the Ranch models (instead of the flimsy flip down sight) and better front sight in the form of the winged front.
Barrels on the new tapered Mini -14s were thicker (.625" vs. .560") but Mini-30s remained the same at .625".
The Tech aftermarket rear sight is better than the new Ruger winged rear sight, much easier to adjust, and adjust in small increments.
Seems like QC at Ruger has gone downhill, they let some Minis out that have issues, easier for Ruger to let the consumer do the QC, and if there is a problem, Ruger can get it back and take care of it.
Older Minis like the 189 series don't seem to have any issues or problems, at least none that I've heard of.