Or do you think in the European theatre over moderate terrain a single gun crew would be sufficient to move the gun a couple of km? Depends a lot on terrain.
Oops, I didn't answer that question sufficiently:
Yes, the crews could move certain pieces on (rather) flat terrain over longer distances (rather kilometers than meters), where then the loss of towing equipment or horses used to set free some additional helping hands.
For instance, the
s.IG 33 (with steel wheels for horse-drawn units and solid rubber tyres for motor transport) was pretty limited speed-wise, the solid rubber tyres (or even steel wheels on the horse-drawn version, wooden wheels towards the end of the war, iirc) and missing springing denied towing speeds over say 16 - 22 km/h (in theory, it seems like quite some troops ignored those restrictions), but also made movement on muddy or soft terrain more difficult. Still, the total weight (1,800 kg) and design/size totally allowed for manhandling them over longer distances, imho.
Despite its weight (3,490 kg), the
10,5-cm le. FH 18 could be manhandled quite well (but slower, ofc), due to its compact design and favorable balance, as you can see on the pic above (last pic in my first post), where it looks like the gun is being moved/relocated by 5 men, only.
For comparison, manhandling the
PaK 40 was a bit tougher, due to its long/heavy barrel, its rather unfavorable design/balance (steel carriage/shield) and its solid rubber tyres, even though the gun weighed "only" 1,425 kg.
The British 25-pounder could be manhandled quite well, imho (1,633 kg), it could also be stripped down to reduce the weight (as seen on the Kokoda trail).
The
US 105-mm M101A1 howitzer had a very unfavorable design, its short carriage beds even complicated (just) rolling the howitzer and taking it on the hook, so I am almost sure that manhandling on longer distances wasn't an option for that gun, at all:
9 men, and the gun still seems to barely move, here, so it wasn't just about the weight (2,260 kg), but also about the design.
The
US 76.2-mm M5 AT gun (3-inch) was a tick lighter (2,210 kg) but its weight/design and bad balance even made relocations on short distances very difficult, where rotating those biggies ended up in a labor-intensive process already, afaik, which resulted in a substantial number of total losses during the BftB.
EDIT: Many 122-mm and 152-mm howitzers and cannons (all nations) weighed over 5, 6 or 7 tons (or even up to 12 tons), even the German 10-cm Schwere Kanone 18 weighed 5,642 kg, manhandling wasn't an option for these guns, anymore.
Only the special versions for airborne troops, which had particular design concepts for weight reduction (eg. shortened barrels, cast aluminum parts, pneumatic rubber tyres and/or mechanisms to break down the guns in several packs - to be carried by pack animals), offered significant weight reductions or improved handling, so that guns could then be pulled by troops (in 1 piece), easily (eg. the 75-mm Pack Howitzer M1, weight: 653 kg), if necessary.
I don't think that there were many bigger leight-weight pieces around in WWII, the Germans worked on several alu-versions of some of their guns, but they gave such projects low priority until Barbarossa, in order to reserve the precious resources for airborne equipment (guns) and the construction of airplanes, they still produced a low number of leightweight gun versions for mountain divisions, paras and to reduce the weight of the s.IG 33:
A mix of steel and aluminum was used for the s.IG 33 "Ausführung B" which was towed by halftracks, but the production had to be stopped after a few hundred pieces (to pick up the original "A" design), because alu was needed for Luftwaffe projects. Very low numbers of "Ausf. C", where most parts consisted of "Elektron" (a magnesium alloy : 90% magnesium, 10% aluminum, which reduced the weight from 1,825 kg to 1,590), were issued to field units for testing, but not produced in significant numbers, afaik, as the gun was not accepted by the Wehrmacht.
Similar projects for other (same or even bigger calibre) guns were shelved to reserve alu for airborne EQ and aviation.
I am not sure if there were weight reduction projects for guns with calibres of more than 75 mm on the Allied side, apart from the pack howitzer or similar concepts.