Armstrong scored 13:14 into the third period and Atlanta beat the Canadiens 5-4 on Tuesday night. His goal came 34 seconds after Montreal drew even for the second time in the third.
"It is a tough building," Armstrong said. "I think it's an exciting atmosphere and I think guys get up to play here, but at the same time it is a nerve-racking place to play. To come in here and play like we did tonight on the road, I thought we played pretty well."
Rich Peverley had a goal and two assists, including his third point of the game with a cross-crease setup pass on Pavel Kubina's power-play goal that gave the Thrashers a 4-3 edge 4:10 into the third period.
"Every time they scored we seemed to come back and have a good shift and kind of get back in the game," Peverley said. "That showed a lot of character on our team."
Tomas Plekanec drew Montreal even at 4 with his fourth goal at 12:40.
Brian Gionta scored his second goal of the game earlier in the third to draw the Canadiens even at 3 after Mike Cammalleri launched Montreal's comeback bid when he scored his seventh goal with 35.7 seconds left in the second.
"It was kind of a back-and-forth game and we just happened to, I don't know, I guess get the last goal pretty much is what it came down to," Armstrong said. "I think we'd like to play a little bit tighter but at the same time it's just good to get two points."
Ondrej Pavelec made 34 saves and Peverley increased his team scoring lead to 16 points. Kubina, Bryan Little and Mark Popovic each scored their first goal of the season for Atlanta, which had two shootout wins in its previous nine games in Montreal.
The Thrashers (6-4-1), who played their third game without injured captain Ilya Kovalchuk, won their second in a row following a four-game losing streak.
Kovalchuk, who leads Atlanta with nine goals in eight games, was lost for three to five weeks after he broke his foot in a 4-3 loss to San Jose on Thursday.
"He's one of the most dynamic players in the league," Peverley said. "We have a lot of depth on our team this year. We've got some key pieces up front, on the back end, and in goal, too, so that just shows you the overall depth we have on our team right now."
Montreal (7-8-0) ended a five-game winning streak at home. The Canadiens had gone 7-0-2 against the Thrashers at the Bell Centre since Atlanta's 4-1 win on Feb. 17, 2004.
Carey Price, who stopped 25 shots, saw his record drop to 2-6.
"You can put on a fake smile and say everything's O.K. but it is frustrating," Price said. "They're not all bad goals that are going in. I'll keep working and hopefully those good goals will stop going in."
Gionta, who scored his fifth goal early in the second, intercepted Christoph Schubert's pass and beat Pavelec for an unassisted goal, his sixth of the season.
Peverley had a hand in both of Atlanta's goals late in the first period.
Little, a 31-goal scorer last season, got credit for his first of the season 17:34 in, when Peverley's shot deflected off his stick past Price.
"It's good to get him a goal and get him on the scoresheet," Peverley said. "I think he's pretty happy. I think he's going to get going now. He's a great player, he scored a lot of goals last year for us."
Peverley made it 2-0 when he scored his seventh goal with 12.8 seconds left in the first.
Popovic scored 7:30 into the second to restore Atlanta's two-goal lead at 3-1 after Gionta scored 2:51 into the middle period.
Montreal drew within one again late in the second. Cammalleri jumped on the puck after Peverley fanned on a pass in his own zone and drove in alone to slip a shot under Pavelec's left pad with 35.7 seconds left in the period.
- Peverley favored his left leg when he left the game briefly after an awkward collision with Montreal defenseman Jaroslav Spacek near the boards 38 seconds into the second.
- Canadiens D Mathieu Carle made his NHL debut after he was recalled from Hamilton of the AHL earlier in the day. Carle was chosen 53rd overall by Montreal in the second round of the 2006 draft.