Thanks to everyone for your replies.
I've done some measuring.
The apertures in the two smallest noses of my new rivet gun, the Eclipse 2800, are 2.35mm. These noses are both intended for 1/8" rivets, which, according to JRP's catalogue, typically have a mandrel diameter of 1.7-1.9mm.
Hence the size discrepancy between the 1/8" rivet nose apertures on the Eclipse 2800 and the mandrel diameter of the rivets I'm likely to use in them is rather large.
By contrast, the apertures in the two smallest noses of my old Talco rivet gun are 1.6mm and 2.1mm. These are intended for 3/32" and 1/8" rivets respectively. 3/32" rivets typically have a mandrel diameter of 1.45mm, according to JRP [see page 14].
Hence the Talco's 1/8" rivet nose [as well as the 3/32"'s] gives a much closer fit than the Eclipse for the rivets I'm likely to use in it.
The only thing it seems to me can justify the apparently larger than necessary nose apertures in the new Eclipse gun is the existence today of outlier kinds of rivets, such as multigrip rivets, the 1/8" sizes of which in the JRP catalogue have nominal mandrel diameters of up to 2.1mm. Maybe in the 1970's, when the Talco gun was made, there simply wasn't such a large range of rivets for any given size and hence such a large range in the mandrel diameters of a single size of rivet as there is today. In the light of this, maybe there is some justification for the 2.35mm apertures in the Eclipse's 1/8" rivet noses.
Coming back to my own everyday needs, though, 0.65mm of clearance for a rivet whose mandrel may only be 1.7mm seems a bit excessive to me; my assumption is that it will increase the likelihood of rivets going in crooked, of mandrels breaking prematurely or not at all during the fixing of rivets, or of breakage of the nose itself. Hopefully, Redsetter's reassurances are to be trusted, and Bernard's concerns about raising the centre of the rivet won't in practice be a noticeable issue.
I could make rivet gun noses myself if I had a lathe, which I still don't. A shame Old Mart wouldn't sell me his Atlas lathe, in spite of my enthusiastic badgering!